Does your cousin ride horses? Hey if he/she does then that's awesome, he/she could ride the pony all the way up through its training. But it would probably be unnecessary to actually hire a little person otherwise. If your cousin want to help train a pony, all the power to them! :)
In Australia we have Pony Dressage. Officially you have to be under 21 years to compete but I compete in unofficial pony dressage with my boy Comanche. It makes it fairer for the ponies because it is hard to score a pony as a horse. Genereally ponies have shorter and quicker strides to a big floaty horse :) I have only gone as far as Novice on Com but I see them going through all the levels with kiddies in the official classes. I think it is wonderful that kids are getting into dressage on ponies that fit them, I think there is only one thing worse "looks wise" than a teeny kid on a big lanky horse and that is someone miles too big for a little pony haha.
As for training ponies, I am helping my mum with the riding part of her little 12.2hh mare. I am 5'6 and weigh 110lb and feel quite comfy on her. I sometimes ride without stirrups on her to get the leg contact I want, and wrap my legs around her if need be, but she carries me quite well I feel.
And this is me on our little old man who is 11.2hh
He feels so small but carries me quite well. He isn't struggling, he would definitely let me know if I was causing him trouble!! ;)
I measured my height last night and I discovered that I'm actually shorter than I thought I was! I'm really only 5'0"! I don't know why that's such a shocker to me... it's only an inch off then I thought I was (I thought I was 5'1") but still. If I'm wanting to work with ponies I guess I should consider that a good thing. I still weigh quite a bit though... I need to work on that. Now what I'm worried about is that if I decided to work with large warmbloods I'm going to seem like a little pimple on their backs, haha