Hi R., it's a sad situation that you are in and I think it would be a really good idea to get a lawyer. In the meantime, do not attack your ex in any way nor fight with him, just start documenting everything. Write down all that you can remember with dates, names and times. Write down when your 2nd child came back to live with you and why. Try really hard not to write down emotional things, just the facts. When you take all this to court with you, you'll be amazed at how things will turn for you. Absolutely no one likes it when you have dates and names and actual conversations written down. Just make sure that you keep your end clean. The reason you want to document the other child living with you is because the judge will see this as non-compliance on your part to the divorce agreement. If you have good reason for your child living with you, you will most likely be awarded custody and child support. I know it's hard not to get emotionally involved, but just think of it as a professional relationship for now and try to stay calm. It will only help you in the end. Go get yourself a notebook and keep it with you all the time. Make sure he doesn't see you writing things down and do not let him know about it. Do not use it as something to "hold over his head". His behavior will change and you could lose out on a lot of things that could help you in the future. I would also keep careful records of school supplies, clothing, doctor's fee's, anything money related for your children. Keep the reciepts as well and make copies of everything before you go to court. Try to keep all the originals and just provide copies everywhere you go.
Good luck with that and I hope everything works out for you,
Y.