Ok, I strongly suggest you go to the court and pay the $5 (or whatever it is now) for a form that I can't remember the name of that states he is past due. You fill out how much he owes and submit it. It takes a LONG time to get to court and once you get to court he will either
1- not show up
2- cry poverty
3- cough up the money
4- cough up some of what he owes
Unless he pays everything, the judge will make a decision, probably that he's in contempt. But then you have to wait for that to go through and that takes another few weeks.
BUT... you must then file another order for the date starting from your court date again. If you don't, they get away with it again and for longer. It's a lousy system and sadly works in the favor of the slackers.
Also, make sure when in court to change the thing to that you want the court system to go after him instead of you having to do this each time. Then you don't have the stress and time wasted. As soon as he's late, CFS or the Child support office will go after him without you needing to do it.
Yes, you do have to let them see his kids. If you're fearful for their well being, you need to be able to prove and then you have to go to court for that too.
You can do all these things without any lawyers and save tons of money. The courts also have lawyers that you can hire something like 15 mins for making sure you know which papers to file and what you can do, etc.
Good luck