Well, I guess I lucked out. I have a David Clark H10-13.4.
DC sells a Youth version of that with a small headband. They even send you a free large headband when the Youth outgrows the small one.
I called them to see if it was possible to go the other way (large headset to youth size), and their customer service said that while they don't have a kit like that, they'd go into their warehouse and just send me the smaller band. Great service and I don't have to buy a headset.
It looks to me like all the child headsets just shorten the band over your head. The ear-cup size doesn't change.
Pink would have worked for my granddaughter, but the converted DC will do too. She'll get an ANR headset when she grows a little. My grandson still has a couple of years to go before he'll need a set.
Thanks everybody for the offers.