If he gets speedy/bucking in the canter only - I’d spend a week or two really working on the kiddos trot work to get her confidence up - two-point, up-up-down posting, leg yielding, cavaletti, etc. I’d also teach her a one-rein stop. Then I’d set the canter up for success, assuming kiddo is strong enough to do it at all. Do a solid 45 minutes of trot work. Pick up the canter in the middle of the long side, canter 5 strides or so, and back to trot. If the pony doesn’t listen, she is approaching a corner she can aim him into.
While kiddo works on that, put the most physically similar (height and weight) pony jock on to work on the issue. Heck, if pony jock rides in the am, let your DD ride in the afternoon.