Our extended family is pretty close and gathers most weekends, and we've never had a period of time in which we didn't have an elementary schooler in the mix. So, since our tradition is to celebrate the "big" kids, we celebrate the "little" ones too.
The tradition is that on the weekend closest to your birthday we have a party for you, and on the weekend closest to your half birthday, the kid gets to pick the dessert, and the we make it "half" in some way. It used to be that we served like half a pie. But now we have too many people so we do things like bake a layer cake and then cut in in half and stack it so it's got 4 layers instead of two half the circumference. Or we bake two different kinds of pies, eat half of each and then give the other half away. Or we have cookies, and cut them all in half, and everyone gets a few halves instead of a smaller number of wholes.
It's silly, and fun, and we would be making dessert anyway so the only "work" is figuring out what half of whatever looks like.