We have a big advantage in the UK of having a diverse selection of ponies and a small child would never be put on a horse.
All I do with small children is to walk them out and about, with me walking and leading them, rarely ever in the arena. I have them hold the reins and teach them walk on and whoa and they can use their legs on a small pony, and just let them learn the feel, find their balance and be comfortable with the pony. There is some correction but only if it is reins being held incorrectly or stirrups to far back on their foot.
I just let them get the feel, all trot - usually only a jog. Is done sitting. The reason for this is because once a novice knows how to trot rise then they rarely ever do a sitting trot. A good sitting trot indicates a deeper and more balanced seat.
When the child is comfortable on the pony, balanced and capable I will pony them from a horse.
I do not have concentration problems because I do not want them concentrating or trying to force themselves into a position which causes stiffness.
As they get stronger, so then they can progress to riding on their own, they know and have learned the basics of stop and start and turning. The are confident and competent to start riding on their own in the arena on a suitable pony.
I also, when they are off the lead, like group lessons. It makes the kids try harder and towards the end of the lessons we will play games, bending, egg and spoon, relays and so on. It makes them ride without them realising it, adds fun and competition to a lesson.