I think this might cause problems with insurance but swap ponies? Match a horse to a rider with the same ability as its current rider this improves their riding skills. If you want a less extreme version let a different rider café for a different horse for the day.
We also do pony parties and we play games like red light green light(green light they have to squeeze their legs and say woah, yellow light they slow down and say slow and red light they stop the horse and say woah),
musical stalls(lay poles on the ground and the spades between them are the stalls, kinda like chairs in musical chairs. Same as musical chairs the kids walk in a circle around the poles and when the music stops they have to get in between 2 poles into a "stall" and stop the horse, if the horse doesn't stop they keep moving and someone else can steal),
We also play Simon says( which is normal to regular Simon says and when the kid does what Simon doesn't say then they simply stand in the middle of the arena until the game is over, you can say touch your nose, hands on your hips, 2 hands on your head, reach for your horses tail, make your horse stop, go over a ground pole, ect ect. This helps with balancing)
You can also do egg on a spoon race (where you have 2 teams and each kid gets a spoon and they have to carry a plastic egg on the spoon and drop it in a bucket at the other end of the arena and after they drop their egg the next kid can go)
Hope I helped!
We do acouple camps at my barn in the summer and while ride you can make little obstacle courses using cones and ground poles and just be creautifve with it.
And when off the horse if you can make games or worksheets to help them remember breeds, colors, markings and moods.
You can also turn barn chores into fun challenges or games.