My daughter is 7 and her hair is too silky and also too thin. I use Johnson's baby shampoo for her. Now her classmates are tying plaits, so she also wants me to tie it for her.
Here are some thoughts/suggestions (my youngest daughter has very fine hair that not even a barrette will stay) some of these may not work but try different or mix:
1) slightly dirty hair will braid better than squeaky clean hair
2) try braiding hair when damp/wet but use hairspray as it drys
3) keep hair as tight as possible
4) in the beginning - use hairspray on roots so start will be easier
5) have you ever tried dry shampoo? this makes more volume in fine hair - spray on roots and tips as clean hair can be very slippery.
6) some people also use Pomade but we never tried that
Since she is only 7 years old maybe just one braid will make her happy. Check these styles on Pinderest with her and see if you can find something to try;
Possibly try one of the shampoos that is supposed to build body?
Spray her hair with hairspray to stiffen it up and then style it.
For her hair in general, you should keep up on giving her trims (it keeps fine hair strong) and also use an all natural mild shampoo like this one:
www.influenster.com/
To do the plaits, be gentle with her hair and give her a fine mist of hair spray to keep it in place.
Use more hair for the braids to be thicker?