I don't know if this can help you, when I was a preteen I started to get periods where I would eat only very "enjoyable" food, like pizza and junk food. As an adult, in hindsight, I can say my mom manged it well.
Eating is not a simple issue, it concerns many personal sides of ourselves, so I try to explain my point of view.
IMO "forcing" children to eat is mainly a wrong thing, but...
I would sorting food in three categories:
food you really like (not always healthy);
food you don't really like, you don't love it but you don't hate it too and you usually have eaten previously (usually healthy food, like some vegetables);
food you hate, disgust you or make you sick (you tried them with always the same result);
Pushing or even forcing someone to eat the last category is really wrong, even really dangerous in many cases, my mom never did it with me and I would never do it to my children. But if a kid or preteen, or even a teen as in my case, is choosing to have a very unhealthy diet, probably because all that he/she wants is only enjoyable stuff (and probably not only about food, but as a general feeling that is shown in food too), maybe just only for a tantrum, power struggle, or just to complain, well, a parent taking the reins of the situation pushing the kid to eat something of the first two categories (to be clear... NOT the third one) is totally right, in my opinion. The preteen/teen is really acting as a young kid, not showing a bit of maturity to care for him/herself, so the parents should take action, it could means set some ground eating rules, temporarily stepping back to parents choosing what and how much kids have to eat, even the "clean your plate" rule and consequent punishments could be appropriate in some case.
Only the parent knows which actually are these cases, only you know the reasons of their behavior, follow your maternal instinct. That was my case and the way my mom managed it, as result I had a really good diet as a preteen, teen and I still eat a lot of vegetables and food I don't really like just for my healthy living, food that otherwise probably today I would never eat.
In short, if you think it's just a power struggle, a step back to childish behavior, or similar things, try to correct that as soon as you can until they show consistently a little more sense of responsibility.