With my 4 y.o. I used the reccomended size 8 needles. For my 10 y.o. I used size 9 and had to make his just slightly smaller than an adults. Since I made them for my kids and they weren't a surprise, I just knitted to the size the pattern said and then had my kids come step on the knitting. If your gift is a surprise, maybe just ask their mom/dad or some measurements at match that up to your knitting. I knitted rows until the were even with the toes, but I think they ended up a little loose. I was okay with that as my kids' feet grow fast, but if you want a tigter fit, maybe go a couple rows short of the big toe so they'll stretch slightly to stay snug. I hope you enjoy them! I just finished making a couple more pairs for gifts for my in-laws. :) I do reccomend metal needles for slipperyness as the double-strands of yarn would get a little hard to slip off my bamboo needles.