I think the best thing to do is to print them out at different scales until you find one or some that fit. Kids' bodies are proportioned differently than adults anyway, so the size of the chest armor might need to be at one scale, and the gauntlets at another. Once you have the scales fro the different parts chosen, make notations of the settings so you can reproduce that on paper or cardstock etc.
For the helmet, same idea but maybe you can get away with taking measurements of the kid's head and size the helmet to fit those. Keep in mind that it's a lot of work and you may want to size it slightly large so that it fits for a few years.