how about looking towards the future - open up a new bank account, maybe have your mums address and just start to put away a little bit of money each month - not too much that he gets suspicious but enough to make a difference over what ever time frame you give yourself. At least then you are doing everything in your power to make a future for you and your children.
As for the custody thing, start making a diary now of everything that he says or does (make sure he doesn;t know or find it - maybe keep it at your mums). Keep on being exactly as you are with him now so he doesn't get suspicious. Don;t let hm bully you and make you think he'll get the kids. What about getting some counselling on your own - it might help to make you stronger for when you do decide to leave him (if thats what you want to do). I'm sure you will get plenty of support on here whatever you decide to do.