Need to now a few good web sites about finding people?

My boyfriend is a very secretive person. I do know that he was previously married with 2 kids. They are old enough to be computer literate. Is there a website that anyone might know of that I can go to to see if the kids have registered to find their dad. You know what I mean, one of those I'm looking for you you're looking for me kind of sites. Like the ones adopted children sign on to to find their birth parents if the parents ever wnat to be found later in life when the child gets older.I really want to reunite him with the kids. I have to search for things under his name because I don't know theirs. Thanks