I have no clue what she is, but do you have any recent pictures of her? I'm curious to see what she looks like when she fills out. If your 6 year old can handle and ride without any problems, I would say she's a keeper! She's got that kind, good natured look to her.
I would just say pay attention to saddle fit, especially as she fills out- the fit can change, and you don't want any pain issues stirring up trouble.