Jet boil 
I dunno how old this niece is, but I can attest that kids LOVE having this experience of lunch on the mountain.
I can't recommend a jetboil though, because the adult would have to be on board of doing this if the niece is young. If the adult is a pack of jerky, couple granola bars, and ding dongs type they won't take the time to use the jetboil, and the niece won't ever get that experience. It'll sit and collect dust.
If the niece is young I'd run it by her adult hunting partner and see if they'd use it...if they already have one and use it, then it's kinda redundant until the niece is on their own hunting.