erm i can see your concern.at the riding school i work at the children an start riding from the ages betwwen 4-5yrs old.but this does depend on the child as most children at that age just want to have a sit on a pony.ive been teaching this little girld called issy that is only 5yrs old for about 4 months and she has progressed really well.she has got her rising trot going well and i only let her canter wen i go around with the pony.and shes goin over tiny cross pole jumps by her self-i started goin with her at first.
so maybe if the instructor goes with her and holds her she should be fine.with the jumping it could be something not much bigger then a pole on the floor but its still a jumpo and you will proberly find that the pony will just trot over it ..but you can still get excited and tell her how well she has done.and that she has just jumped!!
i hope this is helpful to you.let me know how it goes!!!