My daughter helps with feeding (mostly watching, but getting more involved), ATO refills, water changes, fish naming and general watching.
She also went to Aquadome with me to pick out the wrasse we got.
Sunday is her 6th birthday and I will be letting her choose a coral to order in this next Vivid order. It will be her coral and she will get to choose the placement in the tank (within reason).
As far as random "additives" to the tank, I think I may (re)emphasize that in the coming days, just in case. To date she has never really suggested doing anything like that...