Please don't be offended but PLEASE ditch the sweet feed - especially for youngsters.
In this day and age of "everything-in-life-is-vitamin-mineral-deficient, they do need something of quality that is suitable for their age, so they develop properly; sweet feed isn't it:(
Ten acres is a nice amount of land for two horses to play and be horses
I know rounds make life a lot more simple for the human but, I am not a roundbale fan either.
First, two horses can't get a small round eaten fast enough before much of it rots and molds. Sometimes rodents decide to "winter" in a roundbale if there's nothing else available.
Second, you can't control how much they each eat.
We are living in an age where equine metabolic issues have exploded on the scene. These youngsters need their feed pan and their hay controlled to reduce that risk.
If either of them happen to be predisposed to metabolic issues, they may even need pasture time controlled before they're five years old
I hope this helps