How bumbproof is your horse?
One thing i've done with young students is take a nice soft squishy ball and play catch. The 'problem' with younger children is the attention span and simply giving them a spoon and ball, well it gets boring quick. I put my mare on the lunge line with no bridle and play with the child. It works on their balance and how to move independently. They will twist to the side and need to isolate their hips and legs from their torso. They'll be focusing on something that isn't still. You can't space out of much if you're constantly doing something. This will get their coordination better to. Then you can say, 'keep on hand on the mane and only catch in one hand". This will start getting them to isolate their hands from each other too. I do this going both directions at a walk. Once the child can start focusing a little more and is thinking about their hands I move on. Your horse does need to be a saint though and you need a squishy ball just in case one of you has a total fail in playing catch. You'll want to start out in a smaller circle and slowly move out.
From there I set up obstacle courses in the round pen. I have the child weave through cones, ride up to a barrel and pick up an object, take it to another location, mount, dismount etc.
When the child is on the rail I have them do red light, green light to help them along with their focus. I also like to play simon says on the lunge line; ex "simon says reach back and touch her butt" "Simon says keep your hands in her mane for two strides" "simon says arms out like a bird" just make it fun to help her focus.
You've really got to keep kiddos doing stuff or they space out. It will be fun for her to pay attention with games and you'll teach her to get focused without any frustration.