To help children with low muscle tone or strength learn to creep, try putting them at the top of a gently slanting smooth board. Put an interesting toy like a Skwish, at the bottom and watch them go! A couple of hints for kids who are slow to get it:
Bare feet help.
You can place your palms at the soles of their feet to give them something to push off against.
If needed, flex their legs reciprically to help them learn the movement pattern.