just popping in for the usual "only here will people understand the significance" reason.
So DS2 (now 6. 3 but at 3.6 had severe receptive language delay and still has sensory issues and social communication problems) managed not to tell DS1 what his birthday present is for a whole two weeks. He also no longer discloses his hiding place during hide and seek. He seems to understand his own mind a lot more and slowly starting to understand other people's (though we have a long way to go).
And just as good is that I managed not to let this latest achievement of DS2's dominate the celebration of DS1's birthday.