I think their toys also get a second life at that time because they can show off all the neat things they can play with.
Ain't that the truth! DS6 is finally interested in all the toys he never played with, now that he has DD2 to show off to. She likes stuffed animals, so now he has to sleep with a dozen of them himself! She likes his wooden train tracks, so he has to build for her. She gets a kick out of building blocks, so now he likes to stack them up so he can yell when she knocks them over. The list goes on and on.
