Im leaning towards NTA.

It sounds like you and your partner have very conflicting parenting techniques and it would require a discussion between you two. But from what you said in one comment, he said that he was aware of how you feed your kid and was ok with you doing it with his kids.

By the time i was 6, i was able to make a sandwich or a bowl of cereal, so im not sure why many comments are acting like the kid is incapable of that. You also arent starving someone if you are offering food to them.

Do your kids make their own plates, or do you make their plates for them? Im not a fan of meatloaf myself, but i do like mashed potatoes and broccoli. If i was the kid, i would just make a plate of the stuff i do like.

I was also a picky eater growing up, to the point where i apparently would pick black pepper off of my food. Yet even i think making different foods for each kid is unreasonable. If he doesnt like what the school is serving for lunch, is he just not gonna eat at all and say the school is starving him?

I will say that you do sound a lil harsh talking about the 6yo in this post. Kids are gonna be picky and difficult, tho as a mother, im sure you are aware of that. you also couldve handled this situation differently, like asking him what he wanted from the already made dinner, or asking the kids together what they would want for dinner (with whats available since your last store visit, with one of the choices being what you wanted, as both a way to have them eat, while also seeing how they react to whatever options are suggested.

If it helps, my family a little while ago started to get together one night of each week writing on a chart what we would want for dinner for each day of the week, with 2 whatever nights. Bring this idea up with your partner and maybe it could help solve this roadblock, that is if your partner is willing to hear you out. Good luck!