My mum has asked me to help my 12 year old sister to move up to my dad's Mecklenburger gelding, since I can't take him with me to school and my dad no longer rides. For the past few years my sister has been riding her POA gelding (he is a small large pony) on and off, but he is about to hit 20 so all he can really do now is walking trail rides. Charlie (the Mecklenburger) is a complete saint, wouldn't hurt a fly, eager to please, etc. but with him being bigger than Fly Boy (the POA) my little sister gets intimidated. Also Fly Boy is just a plod-along, follow-the-leader so my little sister kind of expects the same from Charlie and thinks that if she just puts her heels in his sides and hold on they will go places, but that's not how Charlie roles.
I'm kind of bad at articulating to her how to get him to do what he needs to and I'm not sure she really understands what I am trying to say, so any tips on how to get her comfortable?