I have this issue with my 12 year old daughter. She is overweight and yet still wants to wear a two piece -- bikini. I tried talking her into the "tankini" but she wouldn't have it. It was a battle of the wills as I tried to discuss and reason with her and very frustrating. I was trying so delicately to explain to her why the answer was no; however, this just kept the discussion going on and on and on. So, I finally said to her, "Look. I am not trying to hurt your feelings. I love you very much. I am your mother. And, as your mother, it is my responsibility to protect you. Kids your age are cruel and it will only get worse the older you get. The real deal is that you are to round and thick in your midsection to wear a bikini. Period. You know what? So am I. And, when you wear something like that with your shape, it only makes you look bigger. I am not trying to hurt you but rather protect you from the ridicule you are going to subject yourself to. That is one of my jobs as a mother...to protect. That's what I am doing. I have tried to be delicate about it and that hasn't done anything but invite an argument. And, besides, if your body WAS one that looked good in a bikini, I would probably STILL have a problem with it as you are too young to be going around boys dressed like that. So, there will be no more discussion about it. Now, if you want a swimsuit, I will buy you a one-piece for now (she needed it for the next day for a field trip). We can shop for a tankini for you later, but you are going to get something that makes you look "classy" and not anything that makes you look trampy. If you don't want a one-piece swimsuit, then fine, you can skip the outing. Make a choice."
Now, it sounds cruel and it felt cruel. It was probably one of the hardest conversations I have had to have with my daughter, but it was necessary. And, yes, we both cried. But, she understood where I was coming from and it wasn't that I was just trying to be "mean" by not allowing her the two piece. She opted to get the one piece and I will continue to shop for her a more flattering two piece tankini.
Hope this helps.....it's such a hard time for girls at this age.
Hope this helps.