What are they eating now? Pasture grazing? Hay? Feed/grain? Recently dewormed? If so, with what?
I agree with free choice grass hay and/or quality pasture, 24/7 turnout (or as close to it as you can get), and deworming. On top of that I would add some alfalfa pellets, flax meal, and a goot vit/min supplement for young horses. I prefer not to use sweet feed or pelleted feed.
Free choice good quality hay and a vitamin/mineral supplement like a ration balancer fed to their needs as far as age and weight :) KISS is what I like the less mixing and matching the better you are .
Watch your Ca:P ratio on all feedstuff to avoid OCD's, Keep sugars low to insure they grow at THIER level and not too fast