I've been enjoying the story so far. You have a good grip on pacing which is good because having Sarah instantly going from sweet mother to ramaging villan would have resulted in a significant drop in quality (people don't exactly become murders the moment they're given the chance). I guess with Clare you give the idea that's she's rebellious to begin with, which I guess would explain why she's so willing to kill after becoming a giantess. I'm not quite certain that Sarah will be completely on board after chapter 5 but having her defend Jason was a good way to get it started. I expecting her idea of "acceptable" victims will become more and more generalized as the story goes on (or not, depending on how you spin it).

I've got a few things I'm hoping will happen (Sarah and/or Clare interacting with Jason's teenage peers comes to mind. Other than that I'm greatly anticipating what comes next.

You might what to comb back through a few of your chapters because I found a few errors (e.g. using "she" when Jason was talking).