Focusing on simple joy:
* Little children were as happy as sunbeams on a summer day.**
* Little children were as happy as butterflies in a flower garden.**
* Little children were as happy as a puppy with a new chew toy.**
Focusing on playfulness:
* Little children were as happy as kittens chasing yarn.**
* Little children were as happy as a bouncing ball.**
* Little children were as happy as birds singing in the trees.**
Focusing on innocence:
* Little children were as happy as raindrops on a window pane.**
* Little children were as happy as a gentle breeze blowing through the leaves.**
* Little children were as happy as a field of wildflowers.**
The best simile will depend on the specific context and the feeling you want to evoke.