Hockey is dangerous and injury prone, it is a contact sport and an aggressive one, at that. Now, there's nothing wrong with danger, as everything has a certain inherent level of risk. The issue of identifying the possible extent of that risk, knowing what to expect, and then evaluating (parent AND child) whether the risk is worth the reward of participation. There are female hockey leagues and I cannot say that, at the amateur level, they are less dangerous than the comparable male league. And it certainly isn't on the same level of risk as, say, driving while talking on a cell or jogging along a road at night. The latter two have much higher rates of severe injury per capita than any youth sport, including martial arts, and yet most people don't think twice about engaging in the behaviors repeatedly.
I add that to say that while hockey IS risky, there are many rewards to participation and a lot of enjoyment to be had, and it doesn't behoove anyone to overstate the risks of a sport while underestimating the risks of just living life. On the grand scale of things, severe injury from hockey at a youth level and even into collegiate just isn't that high. I say that as one of the figure skaters who watched the hockey teams in the other rink, and as a mom who understands the desire to minimize risk for my child. The thing is, short of bubble wrap or complete avoidance there ARE injuries incurred in sports, but that should not automatically discount the entire endeavor, wholesale. A thin girl might be more rattled around by a hit for a larger girl or boy, but she may also be quicker and avoid a fair bit of contact, too. You just never know