If a child was born in the United States, but they moved to Thailand with their parents back in 2000, what are the chances that the children can come back to the United States? The parents were never USA citizens, but they are now Thai citizens though. Does that make the children USA or Thai citizens?

If the child was born on US soil, the child is a US citizen.

So the child will always remain USA citizen? I am asking because, the parents are not able to come back due to having three other children on Thai soil, (they are my husbands brother and sister-in-law) and they are wanting us to bring the children back to the USA for education reasons. Any advise or suggestions are appreciated.

The child is US citizen and he/she can return to US anytime... but the parents might be barred for at least 10 years. They would need to apply for visitor's visa which the chance to get one is very slim when they have overstayed their visas for a long time.