I'm not a parent but I don't see a problem. It's not like you're putting her on an un-broke 2 year old or a green broke horse and/or without proper attire, plus she's on a lead line not left to her own devices. My bosses granddaughter rode with me when she was about a year old, when she turned two I always gave her pony rides on my mare (who's as kid safe as they come). Can't keep her OFF of horses lol.
My brother rode at two on a leadline. And I was also on a horse by two, we didn't have them yet but my mom always took me out to places that had some so we could ride. I think as long as the horse is safe, the child has a helmet on and at that age they're not turned loose that it's all good. Honestly from what I've seen, once a kid has been on a horse and likes it, it's impossible to keep them off!