DC sell a child's headband that you swap out for the standard one. This allows you to convert most DC headsets.
When my daughter was smaller (she's 9 now) we were caught out without a child's headset once. We happened to have a small beanbag and put that on her head before a normal DC headset. Worked a treat!
I'm sure by the time she was 6 she was just wearing a normal DC
The best child's accessory is an adaptor to allow a 1/4" jack to fit to an iPod. My daughter loves talking books
Alan