I made one up for my oldest when he was about 2 and the 2nd one wore it too....
CO the number of stitches you need for half the diameter (like if you want it 30" around, figure out how many you need for 15"). Knit every row until it's the length you want. Cast on stitches for the sleeves, knit a few more inches till you get the length from hem to neck. BO the middle stitches, enough width for their head (if the head measures 18" around, cast off about 9"); on the next row, CO the number of stitches you bound off. Knit the same number of rows/inches you did on the back for the upper body, BO the sleeves and knit until the body length matches the back. BO. Sew up the side and sleeve seams.
Very easy and fast if you do it all in garter stitch.