I don't really think you can do anything nor should do anything to prohibit your adoptive children to question their histories. It's not a bad thing to wonder where you come from? I mean really. Why would this even be something to try to avoid? WHo would not want to know? I know there are some that say they don't care- and that's their choice but from where I am sitting- that comes from fear of hurting their adoptive parents and not really being true to themselves. You know- the kids who have adoptive parents with attitudes like Smithies. the kids that end up in therapy- for years saying my adoption does not matter... my adoption does not matter.... only to find out- in the end- that yes- it does matter.
BTW- I just described my experience. I had issues for years and was in therapy- and never ever thought my adoption could be the root- and it was.
The time I wasted pretending it did not matter is ridiculous.