I don't see anything wrong with buying designer clothing for children - however, with a lot of pieces that are shown, they are so casual and basic that they look just like any other piece of clothing out there.
however, if you got a special piece for your child for certain events, etc. I could understand that. however, normal jeans, and shirts from Dolce & Gabbana for kids, etc.. I just don't get it
although, that's just my personal opinion
if you've got the money to buy designer clothing for children, go for it!