I would say that you need to leave it to him to tell them and then do your best to deal with the aftermath of the conversation.
And get the CSA involved asap, child maintenance is an obligation not a good will gesture.
You know, one of the best pieces of advice I have ever being given is to say 'I don't know' when they ask why the other parent did or didn't do something, Acknowledge their feelings and then try to distract them. There is no point in trying to find excuses for the other parent's bad behaviour which later they can either interpret as lies from you or even worse, make them create an idealised image of a dad who is not there for them and never will.
It is about moving on, accept he's' a bad person who you no longer need to protect or consider his needs. Treat the whole situation in the way you would with someone who has died and move on.
You have my sympathies - I cannot believe how many scummy people there are in the world who behave like these people do
I think you just have to focus on explaining things in very simple terms - my Ex was completely insensitive to my kids and has completely messed up the whole sad situation. OW has now had the baby and the children don't want to know/talk about it.
It's so very very hard, but you are the stable part of their lives and you can be the anchor they will cling to while their feckless father swans off leaving chaos in his wake. Answer their questions honestly, try not to criticise him (THAT is the hardest part!!!) and carry on being the one who picks up the pieces!
You sound (understandabley!) angry with him. Contact CSA, get a contact agreement sorted out. It's sad but he sounds like an arse and you have to accept that, not let him treat you like dirt but accept that he's behaving badly and it won't be a surprise if he carries on like that. So concentrate on setting up boundaries so he knows what the rules are and has to stick to them. You can't make him obviously so have a backup plan of what to do with your children when you work. Build a life for yourselves so you can move on from him. Yes they may well suffer, sadly, but the most you can do is try and handle it as well as you can, you can't control the other parent.
Do you know why he left you and got someone else pregnant straight away? Cos I can't think of a good reason for that. If you do end up being the one telling them just stick to the facts, Dad and X are going to have a baby, you'll live with me and visit dad and X and baby whenever it is. Listen to them but I wouldn't try and offer any complex explainations. Saying you don't know is fine.
I just wanted to say i am going through something very similar, my sbexh is expecting a baby in june with his gf he left me for in October. I have two ds and i am due in 7 weeks with my next. My ds1 (5) was already finding it very confusing that daddy has a new gf, and does not love mummy anymore, made worse by lack of information from daddy. I am just taking the be honest about the situation, answer questions and just keep telling him how loved he is stance. I try to be positive about most things too, but there is nothing wrong with showing you are finding things hard. My ds1 is definitely reacting better to honesty about all emotions, than a constant positive vibe from me. I'm sorry i don't have any advice, i just wanted to let you know you weren't alone.
Mum sbexh left the family home in October.His very,very much younger girl friend is 10 weeks pregnant.How do I explain to my children that daddy left his family to have another one with someone else without destroying them.They love daddy and throughout I've focused on being positive about every thingBut ,I can t be about this.Im so mad at them my ex hardly pays any maintence and took me for a ride financially before he left he can't support the children he has got how the hell is he going to support a girl friend and baby as well.Ultimatly my kids will suffer for this emotionally.I can't imagine ow welcoming my three kids into a 2 bed flat on alternate weekends with a newborn and I have to work my second job then.any advice?