T.F.
I don't have Legos, but I'd keep the boxes and instructions.
My daughter got into Pokemon when she was younger. I have everything with original paperwork. Cards are in an album untouched.
Who knows... Her children may hit a gold mine somewhere between all the Barbie, Bratz, Pokemon, Beanie Babies.... You get the gist.
I have plenty of good storage now but if we ever downsized, she'd have a decision to make if she wanted to keep them or not.
FWIW... I am selling my IPhone 5 32GB and I get more money for it because I have the original box, original instructions, unopened ear buds, power cord and adapter. They are not that hard to keep in my office file. That said, if I ever downsized, I would be throwing more things like that out but for now, I have excess room and will use it.