Children often have very fine hair and a smaller head than adults, and may also prefer a different look.
Braids which use a smaller number of strands usually fit their head better. Fine, short hair will hold three strands very well, but multi strand braids tends to look wispy. Thicker hair will hold other braid types, but it may not be long enough for certain styles. Classic braids and crown braids tend to be popular choices.
For more pictures of braids I've done on children, see my Flickr set here.