Poil de Carotte is a red-haired character from a book, yes. The other children nickname him that, it literally means “carrot fur” 
A muffler is one of those relatively short scarves that you’d typically wear in Fall ~ César has a wool, light blue one that matches his mittens, because he’s never happy with the weather. 
By the way, I’ve been meaning to ask, what does the uniform look like ?