A few drills:
If it is a small child, you can just pick them up and show them the position you want their body in as they are doing the dive roll.
I've also had them run and punch and land in my arms (repositioning them into a correct dive roll position).
We've used springboards to 8" mats to get a bigger punch - just looking for shape here; I know that this does not teach them to punch.
Also with this setup we have set up a barrel between the springboard and 8"mat.
Also with this setup we done springboard, over a barrel, down an incline (to encourage the roll out at the end)
We've set up multiple barrels for the kids to stretch out over as they do the dive roll. We've used many different types of props (foam blocks, stuffed animals, etc) the key is to get the props long enough so that the kid has to stretch out to make it over and clear their feet.