there are so many different wheel makers out there, from cheap toy bikes to ultra-high end race bikes that there isn't a standard chart. since bicycles don't need to meet any DOT or other standards, it would depend on the manufacturer.
that being said, heavy duty 20" rims are easy to find, just look for a good quality BMX wheel. you can still find older steel wheels with 12g spokes and heavy duty hubs. i had an old set of dimpled Femco rims which were as strong or stronger than any Worksman wheel. (worksman still has an 11g steel clincher, by the way.)
you could probably find a good quality set of 16" wheels, too. again, check with the BMX'rs. 16" pit bikes were really popular a few years back, and they made some pretty heavy duty stuff.
i'd stay away from the lowest budget kid's bikes if you're worried about strength, durability and safety. just looking at some of those wheels should be enough to scare you away. those bikes are designed to be disposable, since little kids outgrow them so fast, and the price is so cheap it's not even worth it as a hand-me-down. not to mention a 5 year old isn't gonna be putting much stress on it.
a well-built 20" wheel can take more abuse than a larger, comparatively built 26" wheel, because there's less flex. it's a sturdier, compact design.