The younger kids are my favorite to teach. Always start them on the ground...handling and grooming, let them get comfortable first. No matter how small the pony (or big the horse!), the first time mounted up can be intimidating for some of them.
Make sure they know how to carry themselves safely around horses (where to walk, touch, avoid, etc) and I always let them ride for a full 30 minutes their first session. Get a feel for the horse, learn how to hold the reins, hold their position, heels, etc. stopping, steering and asking the pony to "walk on" are also things worth spending time on. And as for the balance exercises, definitely a good idea!
Sounds like you have an excellent plan...just remember to keep them involved, make sure they understand what you're asking and go at their pace....both for their physical ability and mental ability. You'll do great!