There have always been “designer” kids lines, but lately it seems as if there has been a resurgence among the highest end of the market. I first noticed it this fall while standing in front of a rack of Dior kids at Barneys on Madison Avenue. Every mother of a little girl who entered the section made a beeline straight for a dove gray cashmere coat that cost more than I’ve ever spent on my own winter coat. I knew it was a solidified trend when Lanvin staged a three city tour pop up shop to promote its new line, …