Any person who, regardless of age, sex or stature, adores food with every fiber of their being. Note that being a fat kid does not imply anything about a person's weight; it is purely a state of mind.
Certain moments and activities can be categorized as Fat Kid Moments and/or Activities. Examples: going to the grocery store at 2am because all you want in life are brownies; eating an entire pint of Ben&Jerrys when you really only meant to have a bite; pity eating the last bite/portion of food on a plate/bowl/cake pan; assuming other people's food left in the fridge more than three days is automatically up for grabs; patronizing buffet restaurants more than is strictly necessary/healthy/sane; putting whipped cream on..well..everything; interjecting thoughts about food into conversations that had nothing to do with food until that point; etc.
"So...you going back for thirds, too?"
"Um, obviously. Haha omg we're such fat kids."
"Dude I'm such the fat kid...Everyone at the party had these dainty portions on their plates, and I was wolfing down crackers and goat cheese like it was my job."
"Hey, did you read that article on Mexican sociocultural shifts and the escalating tension between adolescent social groups?"