Use a wide-toothed comb and it's much easier if their hair is combed while they leave in their conditioner for a few minutes.

I agree with the comments above, never use a brush on curly hair.

Also, for taking care of the frizz, try using Argan oil. It's a natural product that has many benefits for hair (smooth, shine, de-frizz, prevents hair loss) and it isn't greasy at all.