What I would advise us, slow it down. Your priorities are you and DC, make sure you are looking after yourself. Don't do things that are overly complicated.
Have you talked to your DCs school told them what's happened? Keep talking to the principle/vice principle and the welfare teacher. When ex and I split the school holidays had just started as soon as school stated again I was straight in and told them what was going on I asked them to keep an eye on my two and to be aware that there might be meltdowns because of what was going on. One thing I will advise if keep on top of school uniform and packed lunches (if they have them). Make use of after school club, mine was excellent, they turned a blind eye to a few accidental missed payments, make friends with them.
Don't worry for the short term about healthy eating comfort food is just that a comfort There were many times when I was confronted with hungry (by the DC's) and looking at the fridge freezer cupboards and my mind went blank so it was off to maccyDs.
Don't rely on your ex, if they are proving to be difficult block em on everything tell them if they need to contact you, then they write you a letter. Don't take an interest at all in their lives they will do things just to hurt you more.
Go and see a few solicitors the first 30 mins are free ask them what the would recommend for a child arrangements order, throw in a trick question like "What would you recommend if I wanted to move the children's residency to a different city?" The answer should be, we recommend that the NRP bear all the costs of travel to the DC's residency and that can be folded into the cao" Then ask them about specific instruction orders as well as financial order. But at the end of the day court orders have no teeth for enforcement unless you ex behaves extremely badly so that the police have to arrest them.
Finally, find a way to take time for yourself, mine are long radox baths, gym during work lunchtimes and early mornings on a Saturday so I can have a cup of tea, watch some grown up junk TV while the DC's are asleep.
Tell work what is going on, you are entitled to parental leave make them aware you are under stress at home. Money, sorry I don't have anything to say, I've completely ballsed mine up, over compensating with the DC's and buying them shit to con them that they are happy, paying for a lawyer and barrister because of the courts and for legal protection, it can be shit scary what an ex will threaten to mess with your head. Mine went to the extream of using my depression against me and saying that the DC's should be put in care as well as making a few unpleasant allegations that had to be investigated.
I won't sugar this, it is hard, it's difficult it's unpleasant, but over time you will get into your stride, find routines that suit you and the DC's and you will be in control of your own world.