The 2 year old would probably love the Potato Head. My 2 1/2 year old really likes him. If you get a box of Potato parts, I have a few pieces of advice:
1. Go to Once Upon a Toy in downtown Disney to buy it. The selection is much better than the selection in Toontown (and Mousegears in Epcot doesn't sell the fill a box Potato Head anymore). If you spend over $50, you get a 10% discount if you have a Disney Visa card.
2. Don't put a potato in your box. It is the same as any regular sized Mr. Potato head from any regular store and the potato takes lots of room that could be filled up with other fun things instead. You also get a few eyes, noses, hats, mouths, and feet with it to pad your collection if you buy the potato at regular store.
3. DO buy arms (and there is a left and right). They are designed to hold accessories and the regular Mr. Potato head arms are not. I'd get several sets in case you have multiple potatoes being built at once. Also, the pirate ears have a left and a right too.
4. If a piece you want is huge and takes up too much room in your box, it is possible to buy pieces individually and then you do not have to get them into the box. They are $2.50 or so if you just buy one piece no matter how large the piece is. There is also a Dumbo ride vehicle that is $5 that you can buy--it is a whole Dumbo and it goes on instead of feet.
5. Have a salesperson help you stuff your box. They are pretty good at getting extra stuff in. I had my box REALLY full and they still got the Dumbo ears on top of the whole thing. Of course, the ears totally stuck out the top and she had to tape the flaps shut since they barely met in the middle, but they put it in that way and rang it up as fitting inside.
6. The pegs are flexible and you can mush them into odd shapes to get more into your box, but I always take all of the pieces out of the box as soon as I leave the store.
I don't know about the ponies, sorry.