Ooops, I thought you were talking about Boy Scouts.

Yeah, for Cub Scouts you can go a looong ways following the recommendations in Doug's Kid's Primer.

A small kit containing a few large plastic bags, a whistle, water, ...

Smaller and simpler means they're much more likely to carry it. That means they're more likely to have it when needed. Simpler means they will remember what you taught them when needed.