Do NOT take her to a specialist, you should not even point out or say anything to her that will make her self-conscious. What you say now will stay with her for life. You can talk about being healthy (you say this to everyone in the house at the same time, do not pick her out and do this) and that you hope the whole family can eat more fruits/veggies every day, etc. Get the Weight Watchers books, start cooking stuff from there for her. Tell her You are going on Weight Watchers and would it be possible that she can support you by doing it together ? See what she says.
Whatever you do don't make her feel bad about her weight, you will ruin her self-esteem and self-worth. She is not obese, slightly overweight, but do not focus so much on this, focus on her positive traits. Also, if this weight issue is only an issue to your (and she is not aware of it) do not do anything other than keep healthy things in your house (and keep the junk out). If she comes to you about this and wants to lose weight then tell her you would love to help and start Weight Watchers with her. It will help her learn to eat correct portions and the proper foods (and easy to count the points).
She does not need a therapist/specialist or whatever else you are thinking. I know of some nutty mothers out there, one of my daughter's friends at school has one, the girl looks anorexic and her mother keeps starving her, never packing her a lunch, telling her she still needs to lose 5 more pounds, all this girl does is talk about her weight and she is obsessed with this. I am disgusted that this mother emotionally has abused her daughter like this. Don't ever become the parent who needs to have her daughter look like a cover model, those mothers should never have been allowed to have children.