I would just go to the movie theater and ask the management which they would rather you do:
1. not see the movie because you need to eat and don't want just junk food
or 2. bring in your unfinished lunch and ungrateful nephew to see the movie?
that way you won't be breaking any rules because the management is allowing an exception.
If my nephew confronted me about it. I would tell him: "you're right, lets not watch the movie and go to the park instead"
but then i'd rather go to the park in the first place, or the library or a museum or the aquarium or the beach or the lake or just about any place besides the movies because I like watching movies at home where you can eat whatever you like.
Though my kids and I did go with my sister in law and her daughter to see the Croods last week. It was a once every other month treat.
when my kids were younger I used to pack baby snacks, a bottle and baby food for them if they got hungry. I think your lunch would be along the same lines because it was things that you couldn't buy from the theater.
I admit I used to sneak candy and snacks into the movies when I was a teenager but now that I think about it and after reading this thread, I probably won't anymore because we DO have to hold ourselves to a higher standard and we should not justify breaking rules even if we don't agree with them.
So I think if the management doesn't mind: its ok to bring food that they don't sell at the movies theater (like sandwiches, baby food, etc) if you haven't eaten and that is the only time you can see the movie, because they would rather you buy a movie ticket and get that revenue. However, you shouldn't sneak popcorn, candy, soda drinks, etc that are sold at the theater because it is against their rules and causes them to lose money.
In addition, I believe you should put all your trash in the trash can as you exit the movie theater. Instead of leaving it in the seats so someone else has to clean up after you.
I believe that is part of the reason there is a NO OUTSIDE FOOD rule. Because they don't want to have to clean up from the spills from those foods.Or have the food mold or stain the chairs.
It always annoys me to see parents let their kids run wild and leave a mess behind them to let another person deal with. If my kids make a mess, I make them stay and clean it up and if they spill something they have to find a janitor and ask them very nicely if they could clean it up for them and say thank you for going to the trouble. Otherwise they get grounded and we don't go to a movie or eat out for a long while.
anyway those are my thoughts on the subject.
Don't beat yourself up over the topic. Pray and ask God to show you what the right thing to do is.
Ask someone at the movie theater which they would prefer: for you to eat you unfinished lunch and buy a movie ticket or for you to not go to the movies at all.
and then remember that not seeing a movie is not the end of the world. you could always take time to eat your lunch while others are at the movies and maybe read your Bible while you wait for them to come out.