FYI you can currently buy PVC free gumboots at Target and Cotton On Kids. They are made from 100% rubber (other than the synthetic lining).
All the gumboots in Cotton On Kids are rubber and about 50% in Target. So look carefully.
The only difference with rubber gumboots is they need more aftercare. You should dry them off and out when finished for the day. Otherwise they perish much quicker than PVC gumboots. (Mind you that's not a problem here as Little B outgrows his gumboots year to year - 2 sizes bigger this year!!).
Nothing like a good mud pie :-)