Visitation with father who now has troubled foster kids

I am going to make this as short as I can without all details so feel free to ask questions. I have three kids with my first husband, I live in NC with the kids and he resides in CA. Just recently he and his wife have gotten their foster license and the foster children that are in the home are 16 and 12 year old boys who got in trouble at another foster home so they are going there. I do not know the details of what they did and that is what worries me. My x has a 2 bedroom home and my 14 year old and 12 year old are girls where is everyone going to sleep? He has asked the kids to visit him for three weeks this summer. I am worried about him leaving the kids alone with these boys. Do I have any legal rights or should I just have faith that I have taught my kids whats right and wrong and let them go??

I forgot to mention the custody papers say he is to have them once a month, he does not abide to that he only sees them about once a year for about 15 days

If your ex has the visitation rights you do have to send your children to see him as difficult as it may be, unless you move to modify custody, and can convince a judge that your ex’s fostering of these children constitutes a substantial change in circumstances affecting the welfare of your children.
This may be difficult to proven given how little information you have about the foster children and the other circumstances in the home.
I would encourage you to have a candid discussion with your ex about the visit and get answers to your questions.

He wont say what they did. Even if he did tell me I don’t know if it would be the truth. I hate to to say it but he is a compulsive liar, and I know many exes say that because they are mad, I am not mad I am glad he walked out, but he is REALLY a compulsive liar! How much are court costs? Is there another way of finding out how “troubled” these kids are?

The expense of a modification hearing can vary greatly depending on the facts of each case. You will need to schedule a consultation with an attorney before fees could be determined.
If you do initiate suit you will have the power to schedule a deposition of your ex, forcing him to answer questions under oath, and conduct other forms of discovery as well to try and get to the bottom of what the true environment is that your children would be subject to living in.