It's a hard judgement call - do you raise weight as an issue because a child is getting overeight and risk calling too much attention to it at a sensitive age, or do you ignore, hope the child will 'grow into' the weight, and risk them being an overweight/obese teen/adult?
It is very tricky ground, and there will be anecdotal and official research data suggesting that both strategies are both the best way to go and also the most harmful!
Only you know your DD and how she is likely to respond to each approach. However, couching this in terms of 'being healthy' rather than loosing weight must be the way to go, as it reducing the amount of high fat/sugar things in the house, eating healthily yourself as a role model, plus incorporating more exercise into your daily routine.
I have to say, I am 5ft2, not a skinny minny, and weigh more than a stone less than your DD at 40yrs+, so if I were in your shoes, I would be definitely be taking some action. Yes, we are all different shapes and sizes, but it sounds that at the moment your DD does need some help with her weight. I was definitely 'fat' at that weight (and/or full term pg with my children!), and I have pretty broad shoulders and hips.
Can you show her examples of healthy, but not skinny women (athletes? Kate Winslett??), showing that you don't have to be teeny to be healthy, in addition to extolling the benefits to your body and mind of eating the right things and getting daily exercise? Tell her how she needs to have a healthy strong body as she is growing up and that you are going to help her with this
Good luck!