I wouldn't even TRY keeping things cold in checked baggage, honestly-- plan for a flight delay or a possible lost bag. If you try to put things into insulated packaging, they WILL get ripped into and checked, I think.
We made sure that we were minimizing that risk of having our foodstuff "rifled" through, in flying to Europe. We made sure that we CARRIED on sufficient calories for 2 days, at least-- and divided up other foodstuffs so that we would have SOMETHING even if one bag were lost or completely destroyed.
Can the adult chaperone carry a single ziplock with a few shelf-stable staples for him? Like Sunbutter and a package of tortillas, or something?