This is a skill that it seems kids stuggle on. Particularly the slower ones.
The problem most the time that i find is that is that the kids will drive the arms back, and before actually impacting the floor they stop them and try and rock their bodies over, causing the shoulder angle to close, and as a result (due to weakness in the motion) the bend the elbows to make it easier to lever themselves over.
Drill that i use are
1- Do it from sitting in pike on a panel mat or 8inch mat with arms by ears, and finger tips touching (and as a coach i am always pushing down on their palms, and pulling hands apart to teach them that you do this and you keep doing it until the skill is over). From there i have them rock backward leading with the arms and upper back and open shoulder angle (of course) up into a candle shape. (have the mat proped against a wall so it dont slip, and you might have to fiddle a little bit with where they should sit and). This teaches them to impact the correctly while holding good shape. Its harder then you think to do, and most will kinda freak out about doing it haha
2- From there have them hit the candle (with the correct form and entry) then then carry then through it while the press down on the floor with the hands (to develop some strength). you can also do this to work on the backward extension roll to handstand and its good for giants etc..
3- Let them do it by themselves. At the start as the arms are going back, have them shoot their feet over more then up. so that its its not going up to much (as they might not have the strenght). Spot them occasionaly shapping them etc.
4- on floor work it into backward roll to pike stand and straddle with straight arms (which as sometimes slightly easier alternatives, but while still looking for the same features as describered> leading with that upper back, keeping shoulder angle open, no hesitation and stopping of the arms before inpact, hollow tight bodies, toe shooting/leading the desired direction.
You can at the same time reduce the matprogressively making it closer and closer ground level (panel matts great for this gradual decrease). Eventuall they are raedy to go to the floor. You might want to spot them there a little for a bit, and then let them at it.
Like any progressive thing, just cause they can do it with the slopes or the mats, and whatnot, doesn't mean that they will do it perfectly their first times ont he floor (even if they do all other progressions perfectly). So thus do some shapping again, spot a few, and then in no time they will be away laughing.
On thing that ALL kids virtually do is they push of their heads of the floor rather then hands (little finger down into the floor). This is like actually putting the breaks on haha,and its a hands habbit to get them out of if they aren't stopped from day 1. For that use the soft foamy ball, or little cube, or whatever under the chin, have them focus on this and feel that they are dioing it. This is a big problem and its also a great error that will lead later in into the backhip (it probably already is), clear hip, giant, virtually anything with this type of movement.
Aside from that for varsitility you can always do it down the cheese mats, up hill (which is a nice drill because it does allow them to lead with the arms, but they will rarely have the strength to do it uphill any good on their own at those levels).
some conditioning for this is have them, lay on the floor with arms on 2-3in elevation and in a candle/hollow shape, and just have them work on opening the shoulder angle as you guide them over into the push up possition, backwards and forwards, have them do the work sets of about 5-6 2-3times a session to just develop that specific strength.
Hope that helps