Earlier quoted context omitted.
I don't mean to be rude but I'm just curious... did you try simply not giving them junk food? I have done some research on this and have not found any good evidence that most autistic children will actually starve themselves for any meaningful amount of time if their picky foods are not provided.
To add additional context to my other reply, I do not find this question rude, but I do get frustrated with people who seem to think our food problem is easy to solve. My wife and I have agonized for most of our daughter's life about feeding her. She has autism and ADHD, and will often forget to eat if we do not work hard to get her to eat. If she eats a food whose textures, smell, or taste trigger her, she will vomi…
When I made that comment I lacked all of this context of being underweight, having already struggled with this for so many years and also regularly seeing all the different types of doctors that you go to, so without that information I think it's easier for people to jump to conclusions, but I understand it can be time-consuming to add all that context every time you want to comment. As you can imagine a lot of people are quick to call out things that might look like bad parenting when they assume none of that context exists.
Good luck!