Very nice filly!
But, wait a minute.... the seller told you that "kids can hop on her bareback".... are you talking about Bria, or her dam? I sure hope it is not Bria....she is way waaaaay too young for any weight at all on her back. If it were me and that was the case, I would be asking the seller some very serious questions regarding that.
They really do not get a sway back from being ridden too young (sway back is in older horses), but the legs are what is the issue if ridden too young, it is way too hard on the horse's legs/knees.