I was your daughter. Slim and very active as a young child, then I started putting on weight when I was about 7. One of my most vivid and unpleasant memories is of my mother saying to me, "Smokey, do you REALLY need to eat that?!". I was about 4 at the time, not overweight at all, but it stuck with me and her hawk-like watching of my food intake until I left home has damaged my relationship with food for life.
I hit puberty very early - coincidentally (not!) right around the time I started putting on weight. I had breast development before I was 8, b-cup breasts and wearing a bra at 8, then got my period at 9. I was no longer 'the fat kid', but I have broad shoulders and big hips so I was always solid. I was also taller than every single kid in school (and a couple of the teachers) by age 10. Despite not being overweight, my mother's diet culture continued. I developed anorexic and bulimic behaviour in high school, then later on binge eating disorder.
I was diagnosed with clinical depression at 13 and, later, bipolar II. Taking antidepressants and antipsychotic drugs puts on weight. A lot of it. And I've struggled with my weight and disordered eating ever since (I'm now 53).
The first thing to check with your daughter is, is she mentally healthy? Hormones and puberty do awful things to your head. If she's secret eating, she already thinks she has a problem and somebody or somebodies have said something to make her feel that way. A mentally healthy and confident, loved daughter is much more important than a slim, miserable one.
And never, ever demonise any food. Healthy or otherwise. You will do lifelong damage.
This is a very simplistic comment (I could literally go on about diet culture and mental health forever!). I just wanted to say to you that addressing her mental health should be the first thing on your list. Think about whether she's exhibiting signs of early puberty - her hormones will be going berserk a good couple of years prior. Ask her whether she's noticing changes in herself, physically and mentally - I wish to God someone had asked me, because I had no idea what was going on and felt horribly ashamed by things I didn't understand. And I ate to ease that shame.
Good luck to you and your daughter - I wish you both luck and love.