You can do it! 
You don't need to be 'grateful'. They are his children. He isn't doing you or them a favour. You don't owe him anything. Be wary of this anyway. They often use children and given that it was fathers day it wouldn't have been very good for his image if he hadn't had them as being the doting dad, kind, respectable person etc to the outside world would it? The issues are still there in the sense that it is all on his terms and he can change the goalposts at any given moment. I hope you know I mean this in the kindest way. I would hate for you to get hurt if he changes tact again just when you're starting to feel more positive. It's self preservation more than anything xx
The Freedom Programme will help a lot with that to help you detach and ease how you are feeling. It will take time but it will help.
If you can, surround yourself with people who are going to support you and give that reassurance when you're feeling down and doubting things. This is what you will hopefully get from your meeting this week if you don't have anyone personally who will do this. Be really honest with them about all the historical abuse and current.
Seek that legal advice and get yourself in a better position. This will pay off for the children as they get older and especially as they start school. They need stability so something that is better suited to their needs would be great. Get it nipped in the bid whilst they're still young so you can all move forward and know what is happening day to day. Get rid of the worry as to whether he will return the children or not. If it was the other way round, he would be down on you like a ton of brick with the law.
Focus on yourself and your children. Don't let his actions overshadow being able to enjoy them whilst they are so young. They grow so quickly! Look at finding a bit of work. As said before, it will do you the world of good to have you time and focus on something else. At the moment, this is all consuming.
It's so cliché but with the right support, it really is just a matter of time! The more support, and the more knowledge you gain, the easier things will be. It won't always be plain sailing but it won't be anywhere near like it has been.
You'll manage to do that for your children I have no doubt. It's also learning about the early warning signs of an abuser which will enable someone to walk away before they become too involved and no longer can. This is something they teach in the Freedom Programme. The knowledge you gain can be used for yourself and others. It's invaluable.
So good to hear you feeling more positive. What date is your meeting? Xx