The Forgotten (2004)

The children were taken by aliens when they left on the plane to go to camp-- and the characters talk about the aliens conducting tests, but it was not the children who were the subject of these tests. Telly was one of the subjects of an elaborate experiment conducted by an alien who wanted to study just how strong of a relationship a parent has with a child and whether a parent could ever forget the existence of his/her tie. Telly was the only parent who never forgot her son. The alien finally reveals the scheme to her and then tries to take her last memory of her son away from her-- but fails and his experiment is forcibly ended by (we can only assume) the alien bosses who literally "pull" him from the project. Telly returns to her neighbourhood and finds her son and everything is as it was before her son disappeared.

Veronica