I'm pleased you said that, because we also have a mixture, but most of the sets that were built were broken up and are now in communal boxes. This seems pretty unusual compared to most parents, I guess it's mostly because the parents don't want things to get mixed up. In theory we could rebuild according to the instructions, but in reality it's never going to happen as it would take so long to find all the pieces. Almost every day my kids (Boy 6, Girl, 8) do free play with the mixed lego, sometimes for hours. I'm really pleased with it, as it makes a good break from Youtube etc.