Hi @Scaredashell13 Sorry it's such a long post. Hope there's useful ideas in it.
I'm both 'care' and 'parents in prison experienced' but a long time ago. But plenty of more recent experience of others going in and out of care and prison, and or managing to just avoid it, and some of looking after others children while their parent was inside.
This is legal board, but you asked about talking to your children.
So this is how I would be thinking if I was where you are; and you need to start pretty soon given the time scale. *
I'm making the assumption that if worst case happens, kids will have to go to foster care ie: you don't have anyone who can look after them.
Talk to SS about the process and how you could make it less disruptive if it came to it and what the financial situation over your home etc would be.
Contact prison families associations about the same. Know as much as you can.
It will put you in a better position when /if the conversation heads that way, or simply needs to happen with them.
I wouldn't tell the schools exactly what the position is, unless you have an excellent relationship and trust, until and if it's needed. Then I absolutely would.
I might let them know there where some legal issues going on, but I'd be cautious for now. Keeping normality while possible, is in everyone's interest (unless they're acting up in school etc)
Some of my experiences may not help your deeper emotional state, but they may help you in talking to your children, managing their internal reactions and seeking/ directing support towards them,if you do have to serve time.
I suspect like many, and sincerely hope, you wont be going to prison, so be guided by finding the near impossible balance between keeping them in the loop, but not overloading them with worse case unnecessarily, or accidentally using them as a sponge for your distress especially about a temporary break up of family, should it happen, and yet preparing them so they are more ready if the worse happens.
Keep in mind at all times that you are trying to do what's right by them and their feelings and insecurities around the future, and not allowing it to become about you and your feelings, even if you do just want to hold onto them and cry.
If they don't know you had (have) a gambling addiction, then IMO it's where to start:
You need to let them know you have/do have one, and that you've taken steps to ensure you are in control of it.
I would currently tell them that during the period you didn't recognize your addiction, you used money that you shouldn't have, with some part of your addicted brain excusing it, thinking you would be able to win and repay it, but you can see now that of course that wasn't realistic thinking and it was stealing and that's what you have done.
You are in trouble for it, facing up to taking responsibility for it, and are in the process of sorting that out.
I would not talk about prison at this point, I'd let them just absorb this set of facts first, and if you can, leave the rest for a few days. Doing it in stages if possible, is better for them than the big reveal.
They, especially the 15 year old, may ask there and then if you will go to prison. If that happens I would say you absolutely hope not, as you are paying it back, and that 'we will cross that bridge if and when we come to it.'
If they are insistent, then tell them you're in the process of sorting that information out, and you'll talk to them asap about it. But tbh if the whole prison convo, can hold until they've absorbed the first bit, it is likely to help them.
Try to keep it all 'relatively' light, rather than 'a great confessional' or the terror you feel and say they can talk about it with you anytime.
Have something planned for about an hour and a half after telling them, (car journey is a great place for follow up questions, if you have access) but give it up if needed. Be guided by their responses, including if they are more interested in getting on with whatever was pre planned in their heads.(doesn't mean they don't care)
Accept that they may want to rush to tell their friends, (or go into deep secrecy) advise them not to do so as a knee jerk reaction, as we can't un-tell things, and friendships change, but telling them they mustn't, sets up barriers.
Remind them that whatever happens, this too will pass. Whatever troubles it brings, there will be a future where this is not the huge heavy important thing it feels now, whatever happens. Teach resilience by showing resilience but be ready to catch if they find it all overwhelming.
I think it's reasonably unlikely you will go down, but if it's looking towards a yes, I would have thought you where in a you're in a very good position to ask for sentencing to be adjourned, if needs be, be tagged, and given a couple of weeks to prepare your children and sort out places for them to go, pre sentencing. *But I'm assuming there would be an adjournment for a sentencing report anyway.
My own childhood experiences: I didn't have a loving relationship to be disrupted, but I still 'belonged' and to some extent depended on the concept of parents and their 'status.' It was also a long time back and hopefully some things are managed better, but I suspect the impacts remain in some forms and some levels.
My father wasn't in my life, but when I found out he was in prison it made me feel very separated from others and caused me to reevaluate who I was, and how much of his traits I'd inherited and therefore in my mind where 'not fixable.'
I literally downgraded myself in my own mind. No one knew or how badly.
It was the start of seeing that I too could and probably would, be 'prison fonder. The sins of the parent and all that... 'Care' made it worse.
These days the potential of targeted counseling could help stop ideas like this from forming.
My mother's later disappearance was glossed over, dismissed, different excuses given, and not talked about, but I was constantly reminded of what was having to be done for me because of whatever was going on. It was very damaging and made me less likely to tell things I needed to or seek adult support.
I spent much of my childhood and adulthood not being able to really tell what might be real or not, because it was all dependent on what others claimed and what I might have done as to what slipped out. I used to hope that really she'd abandoned me to marry a millionaire!
Clarity, honesty and keeping it simple along with ensuring I was neither held to blame, nor turned into something to be gossiped about pityingly, would have been hugely beneficial.
Prison visits and most of all searches as a child, left me a third class citizen without rights. (also a couple of downright violations but I didn't dare tell)
Looks of pity from some staff and attempts to normalize it by others visiting, did little positive, but part of that was because my mother wanted me aware that I/they (and their crime) where not like them, and I should view them as lower than 'us.' Hierarchy of crime got imprinted early.
It became another forced to be different, even within the other different ones groups, yet it also re-enforced the idea that I was 'natural fodder' to end up in crime and inside. I managed not to, but prison's not the proper deterrent it ought to be, because of how I grew up.
I'm afraid the other thing I have to say, is your parent being in prison makes you incredibly vulnerable to predatory others of all kinds, including the predatory do-gooders, and gossiping staff, most of whom can't but help share the situation around.
This is the stuff you need to be a strong mum to them over and help them combat and if need be, slough of.
I've steered an awfully leaky, battered ship through a lot of storms, but I think my children, and a couple of 'extra' have done ok because I've tried to keep that balance of honesty while remaining adult, not telling them too much more than they need to know until they need to know it, but with enough time to absorb it and discuss it, and done all i could to lower and protect them from all sorts of dramas. Ie; We've been evicted (squats for homes) a lot, but they never saw the ballifs etc we moved quietly like any family moving home, they never saw the inside of any benefit office, or sat waiting outside a court etc, never allowed to see me looking unkempt in hospital, all those things go a long way to creating security in an insecure life. A late to the table one: look after yourself too.
All this stuff is wearing on the body and brain. Hope things turn out ok.