The hot dogs are skinny, but they won’t care. They are located behind Sections 114, 121, 408 and 421, including in the upper level food court.
There are four stands that offer smaller portions at smaller prices for kids. Unless you’re coming from there anyway, I would just put water and peanuts in your bag. Main Street in Flushing (one stop away on the 7) has several great ethnic takeout joints, but this is a lot of trouble just to save a few bucks. There aren’t many takeout offerings a short walk from Citi Field, but there are places near 7 train stations everywhere. The Mets don’t list food as a prohibited item, but you’re only allowed one plastic bottle of water or soda before the game. You’re allowed a 16*16*8 soft bag, and no hard bottles or alcohol. Yes, you can bring food into Citi Field and avoid the inflated Citi Field menu prices, but there are limits. Citi Field Food, Part 9: Bring Your Own Food + Other Money-Saving Tips Some good and cheap takeout here, but not worth the train ride or walk just to save a few bucks.