If people wish to exclaim, then they should be free to use as many exclamation marks as they wish (!)
As a self-confessed pedant, I'm not that keen on the use of said punctuation device where a full stop would have served better, but applying such stringent rules to young children seems very wrong. Getting them to express themselves freely, and to use punctuation as well as they are able seems key, then gentle revision and correction of their written texts will usually get them into the swing of things.
I know that publishers are somewhat 'anti' the excessive use of exclamation marks, terming them 'screamers', and I have seen many manuscripts returned from editing because they have been overly employed - but that's professional writing for you.
In our everyday correspondence, we should do as we please, and even, if we wish, use the wonderful, explosive and expressive INTEROBANG !?!