my two DCs are always having a laugh (DS 1, nearly 7, DS 2 nearly 4) most often about something really eccentric/toddlerish that the smaller one has been up to.
Often DS1 says, what if a grown up did that? (having seen DS2 stand up on his chair at the dinner table and wiggle his bum, or gargle his drink noisily?
what have your DCs done to make each other laugh?