Kids: Do "gifts for Santa", on Christmas Eve, they get to pick which toys to give to Santa, & leave the sack out along with Santa's cookies & milk. This makes them feel good about giving, & makes space in their room for the new.
Adults: Remember FlyLady's rule, 1 New Item In, Take 1 Old Item Out. That's only if your house, closets, and shelves are already decluttered. If they're crammed full & you need to declutter, however: 2 Out for each 1 New In.
If the kids can give, so can you.