i've always said that unless a child in participating in a royal wedding there is no reason why their attire can't be 100% cotton.
for example take the dress I made neela for my wedding this spring. it was perfect for the the formal occasion:
by the way fantastic parenting omkar.
but as it's 100% cotton (and thus easily washable) it's continued to get use as an everyday dress.
especially when the everyday activity is playing in the bouncy house!