I tend to think the 55 gallon trash bags are much better than the emergency blankets - even for adults. Two trash bags can cover a person from head to toe - though there could be some condensation issues :-o
I'd recommend kids carry at least two of them, preferably three - with one pre-cut with a face-hole (no arm holes) at one corner. The hole should be on the side - not the bottom - from one corner down to be just enough to see and breath out of. The idea isn't to stick a head through the hole, which would allow precious heat out of the head. A second one is great for sitting on or in.
I found big 55 gallon see-thru trash bags at Home Depot. I teased my kids that they could use that and then not have to put a hole in the bag to see through it ... at least until the air ran out. Truthfully I've been thinking of trying to use these as transpiration bags to gather water, but haven't gotten around to testing them yet.
If you haven't seen Doug Ritter's Kid's Wilderness Survival Primer its worth a look:
http://www.equipped.org/kidprimr.htm
Regarding your kit, you might add a small flashlight. The little orange LED lights with switches from
http://www.countycomm.com/orngled.htm are fantastic for only $1 each!!! Get a pile of them. Hang them next to the whistles.
By the way, the best low cost kid's whistles I've found - for the $2.95/dozen - are here:
http://tinyurl.com/2b3p2e (they're from
http://www.orientaltrading.com - item IN-20/45)
They are amazing tough and loud, and the wrist lanyard is real practical.
Source for orange bandannas:
http://www.wholesaleforeveryone.com
Note that there are two different 22"x22" size orange bandanas - one is cotton and the other (American-made) is a poly-cotton blend. The poly-cotton one is noticably brighter orange, but not quite as comfortable on the skin.
Source for orange trash bags:
http://www.spectrumbagsonline.com/categories.php?cat=7