ADDitude has several articles on appetite loss in kids with ADHD. It’s a very common problem.
There’s also another discussion thread on this very topic here: https://www.additudemag.com/forums/topic/food-recommendations-for-kids-who-wont-eat-on-meds/.
We had this problem with my son when he was younger (puberty seems to override it and now he eats like a horse). Here are some things I did to combat weight loss when it was an issue:
1. Protein shakes for breakfast—made them with whole milk, whole milk ice cream, peanut butter, and protein powder (watch the sugar). If your child will let you, add a banana too.
2. Add mayo, sauces, and gravy to food as much as possible.
3. Use full-fat versions of milk, butter, mayo, etc…
4. Protein bars or similar are small but can pack a big nutrient punch (read labels).
We tried this for about a year, it kept him from losing weight, but he still wasn’t gaining. So his doctor prescribed cyproheptadine, an old allergy medicine often used with ADHD meds to stimulate appetite. He gained at least 10 lbs the next year and was no longer underweight.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism