This is an easy pattern, it only requires a lot of patience (800 stitches in the end)!

Concerning size/yarn weight: actually I’m not really familiar with yarn sizes (plus the yarn I used had lost its labels), but my gauge was 16 stitches for 10 cm/4inches, and it fits my (rather thin) 8 year old.
I suppose that with a bit of math anyone can work out how many stitches to cast on for the waist ;)