Depending on what is necessary to keep the pony easily controllable, I would look for a snaffle of some kind, with a wide, possibly rubber or plastic, mouthpiece. Either unjointed or with a french link would eliminate or minimize any "nutcracker" action. I think something along those lines would be most forgiving on the pony. I've also seen hunter ponies go really well in kimberwickes, but these are a little harsher in inexperienced hands.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown