I love this question and reading all of the responses! I know I am late here, but I just got back from vacation, was sorting through messages and saw this one. I could have written it except that my son is 9. I have struggled with whether we should try to keep kits separate or not and thought maybe it was best not to be paranoid about it because it is more creative for him to mix the kits. My husband would like to try to keep them all separate. At this point they have overtaken a room and it would probably be impossible to keep them all separate. After reading these responses it is nice to know we are not alone and I think I will let the kits get mixed up, but might try to sort them into bins by theme (Bionicles, Star Wars, Castle, etc.). Thanks for the question!