Agreed with maura.
Her hindquarters and hind legs are a major weakness in her overall conformation - her hocks look rather posty, and her pasterns are a bit upright. Her back looks rather long, and something looks off about how her back ties into her hindquarters. This factors into her ability to track up and collect, as well as the overall "structural strength" of her back. In the pic of her colt, it does look like the long back did get passed on, but its hard for me to tell with the tack on. One more thing that I see is that her neck ties in rather low.
I do like her bright eye and kind expression - she looks like a real sweetie! She just has some confo flaws that, while they may not hinder her now, are not going to contribute a colt/filly with excellent conformation.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown