Right now your 2 year old is taller than the 3 year old so one would be safe to assume that he will mature taller. Hopefully he is a gelding. I might think he will go to 14.3 hands or so. At 3 your mare is fairly small. I would be surprised if she got to 14 hands. Are they of a stocky build? Horses can mature through 5 years. There can be exceptions but I have not seen much growth after 5.
A good feed is always a good grass hay. Be very careful about supplements. Like with people it is easy to spend a lot of money on stuff not needed. There are pelleted feeds for youngsters and a good vitamin supplement if needed. It would be good to have a vet give both horses a good checkup and a recommendation as to what would work for them. They need good feed for growth and development but just don't overload 'em.
Your mare is small. At 51" she would be 12.3 hands at 3 years. Keep in mind you are adding the weight of a saddle and pad (and whatever else you add on) to your weight. A very general rule is 20% of the horses weight as to what they should carry. And it is best to keep it light on youngsters. Your guy has another year to go before you ask him to carry much. Anyway, if you are not a heavy rider I would think the mare can get out on the trail in the spring as a 4 year old. Work up to distance. Don't make the first out a 4 hour ride. Condition her and she should be good. And never feel like a woosie for getting off and walking for a few minutes to give her a break.
I hope you can find other horsemen around you. It's good to have a posse!!!!
Moral support and all that.
Good luck with your new friends...and Merry Christmas.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger