That's not too young (depends on the child of course - one of mine had co-ordination problems so couldn't jump at 4).
Seriously forget the hand under the saddle - just go straight to the scooting option. It gives them the chance for them to learn the skills of braking, balancing and turning. They can still get up good speeds on it once they have got it and then it is just a case of introducing the pedals. If you do the hand under the saddle they have to go faster to balance so it seems to me and they don't cope with the brakes as well. Plus you have to be available to do it - scooting they can do by themselves with a bit of supervision.
Thanks, I'll give that a go - on the grass to start with. How long between scooting and putting the pedals back on?