Oh, the infamous runaway pony - I know myself and both of my siblings got to experience that at least once (if not more often). It's scary, especially for a child who is already dealing with some fears riding. The first time my sister got bucked off her pony (he took off down a steep hill then started bucking

) it took her some time to really get her confidence back.
Like others have said, we went back to basics, in fact, for some time we actually led the pony every time she was riding. She learned to control him, and relearned many of the things she technically already knew. I think it really helped to reinforce the skills, build physical strength, and mental confidence.
Yes, that turd of a pony bucked her off after that, but by then she knew she could get back on and take control of the situation. She also knew some ground work she could do to right away let pony know such behavior was NOT going to get him out of work or doing what she wanted.
Another thing I think that helped my sister, and me for a long time, was learning to fall properly. Granted, I was a kid, but I actually practiced "falling" off my horse in our soft sand pen - even at a canter at one point (not overly bright, but I did it). I had my sister practice this at a stand still and a walk. You have to be careful of horsey feet, but it makes the idea of falling off much less scary, which can be an amazing confidence builder.
I wouldn't suggest doing that with movement, only from a standstill, but you can also learn how to fall properly from your own two feet. Doesn't make coming off any less scary when it happens, but afterwards it's much better because injuries are MUCH less likely to happen!