I'm with Dreamcatcher.
I'm a mom to an ornery, horse lovin' 6 year old girl. She is by far the best desensitization program! Bikes, hula hoops, jump rope, pool noodles, umbrellas, baby stroller, wagon, power wheels mustang, anything noisy she can get her hands on pretty much, they've seen it. She plays free will with anything of her choosing in the barn aisle while I work green horses in the indoor arena. In the outdoor she has her area (a corner blocked in with round pen panels) and spends her time flinging sand with her shovel and singing or yelling or....you get the picture
By the time I send a client's horse home, she's pretty well made them unflappable