As a teacher I spent literally ages trying to get divorced parents to check their children's schoolbags, as I was frequently told that books, homework, essays etc had been left at whichever parent's house the child had just left.
There is no generally agreed etiquette here, only a minefield.
I agree with the poster who said that once you give a present, it is up to the recipient to decide what to do with it. If the children's mother is selling toys you spent good money buying, I understand why you might want the toys left at your place, but please, do try discussing with the children's parents whether they feel the children have too many toys that are never played with.
Otherwise stick to small presents and the savings account idea, or stipulate that a present is to remain in your house before you hand it over.