Hi Emily,
Could you tell us all a little bit about Fergusson:
What type of pony is he, what sort of bit have other people ridden him in etc.
My sister learnt to ride on my old little Welsh pony who would have been about 10 at the time, he was responsive but naughty. We had him in a plain snaffle but found that she lost all control if he decided to take over. We changed him into one of these
http://www.thathorse.com/listing/bridles-bits-and-reins/bevel-bit-stainless-steel/8697/ (i think you can get them with a link in the middle too) and found that the extra poll pressure, although small was just enough to make pony listen more.
The other thing I've heard of kids using if parents are worried about them being too harsh is reins with elastic inserts in them before the bit, they seem to dilute the pulling effect between the hands and the bit.
Hope all goes well, great to see kids still wanting to learn!