I took a chance and paper mached my 4 yrs olds pep helmet. I was impressed with the end result. I was cautious of the cardstock getting soggy and deforming and this did happen but I expected it and didn't push it too much in any area if the deforming was getting uncontrollable. One positive side effect of the soggy cardstock - it allowed me to soften those pointly spots on the top of the helmet. I understand that these boxy edges require sanding in the regular fiberglass process.
I bought a little square $5 tub of wallpaper border paste from Lowe's and diluted it from an Elmer's glue to a cream constistency.
My 14 yr old is now mache'ing his helmet. I did buy the resin and will probably still paint it on the kid's armor, but I feel more comfortable with less toxic paste that the kids can work with.
There are some finer points for getting good paper mache: small pieces for tight spaces, narrow strips for complex contours, vary strip directions for strength, ...
I will post pics later
-dwoo