Is there a schematic that shows you the width at the top of the sleeve? If so, you'll want to make the armhole opening on the back and on the fronts about half the width of the top of the sleeve . (So if the sleeve is 10" wide, start knitting flat at 11"-5"=6".) If no schematic, you can calculate the width at the top of the sleeve by figuring out how many sts you have after sleeve shaping and convert to inches with the stitch gauge. If the armholes are too small the baby won't be happy; if too long you can always do a short seam at the bottom to close up before inserting sleeve.
It looks like a fun way to use up leftovers.