How old are the kids?

If in their teens I would say hands down center the kit around a palmer furnace, if you're traveling by snow mobile then you're in an environment where exposure will kill you faster than anything else. Obviously there is a burn risk and an assessment should be made if the kids can handle having a candle between their legs but for the size and weight I can think of nothing else that will provide lifesaving warmth in freezing temperatures with little to no prep. I'm assuming these will have to be made for lots of kids so cost will be a factor.

There are a bunch of ways you can do it to save money, from contractor bags (black or orange) on the low end to heatsheets emergency blanket + disposable clear poncho or if we're going premium then an SOL emergency poncho. Cheap candles from the store or tacti-cool exotac candles. You can even make your own inexpensively buying tins with lids in bulk quantities and melting beeswax in it with 2-3 wicks. Throw in matches or ferro rod, with a little cotton ball and Vaseline or tinder quik pre installed around the wicks and you can light with a spark. The supplies can be had for cheap enough they can have two, one to practice with and one to use. If you opt for contractor bags then two bags would be ideal, one to throw over them (and rip a face hole) and one to stuff with evergreen boughs to insulate from the ground.

Beyond that, maybe a couple of toe warmers, a whistle, signal mirror (can be had on aliexpress for $1 ea, can improvise with cheap dental mirror from most stores) and something to treat water. Some duct tape around a card is cheap, some bankline cordage. A $1 headlamp from walmart. You can vacuum seal the lot or use a ziplock baggie depending on expense and time and ziplock can be your water container in a pinch, marked at a quart / liter.

Not ideal but If it must be worn around the neck, can punch holes in the ziplock above the closure for a dogtag like chain, or duct tape to reinforce. or put the entire assembly in a kerchief, fabric pouch with drawstring closure, cargo pocket cut out of some old cargo pants, or even a pair of wool socks to help support it so it's not tugging on the freezer bag. I reckon it would be manageable weight wise.

If you want to splurge, a Coghlans pocket sierra saw is a pocket knife sized saw for $10, doesn't weigh anything and would fit in the ziplock with the rest and be fine for gathering some fire wood or assembling an A frame shelter, if help is more than a day away due to weather they'll want to convert to a different shelter with a campfire since the candle will probably be done getting them through the first night.