I never thought this would affect my family. We eat healthily, are very active, good habits etc... and if anything, I had to deal with the opposite with my eldest who was quite significantly underweight for quite some time, to the point of being referred to a pediatrician.
DS was a fat baby, then became slim and ate well. Loved his vegetables, not picky, all good...until Y6 when for the first time, he came home after school rather than going to afterschool clubs and he raided the cupboards. Tried to stop buying unealthy food but still manage to cook himself something and then had dinner again. I was shocked when a year later during summer holiday, I realised he had man boobs and fat all over.
My approach was not to tell him off, take things off him (I knew that he would just use the money he got at Christmas/birthday to buy rubbish on his way back from school) but to talk to him. Inevitably, he got quite upset the first time I said to him that he had put on too much weight because of what he was eating secretly, even though I did so in a very kind way, but after a few days, he himself said he didn't like the way he looked and wanted to eat better.
It took a couple of years, with months when he was committed, some he wasn't, but gradually, he of his own doing, came up with strategies that worked for him. He is now 14, very slim, athletic, and in control of his eating habits. I'm very proud of him, but I also know that empowering him in taking control over it, even if it took some time, rather than taking control of it myself was the right way. He knows he will always have to watch what he eats because he is prone to gaining weight, but he always knows how to go about it.