From personal experience I tend to preserve at least a playset (4 copies) or two per each card of a set I intend to complete, collector point of view, and also you don't know which card would be useful in the future.
Also is true that storage is a problem, even more when you buy different boxes of a single set, I solved the problem making fun decks with common/uncommon and rare cards that I keep in the store where I play at arrangement for anyone who would give pokemon a try or better, for the children that want to start playing and have no idea how.
Anyway I imagine that you, as a father, do not have the time to do such a didactic role so my suggestion, if you want raise some (very little money), make some fun deck and sell it!