When my grandsons was born I made them a similar jumper but it was knit top down and only came in a newborn size. I've been asked so many times for a design like this by my customers but knit flat on 2 needles and in larger sizes too. It's a unisex design that wraps over completely at the front so very cosy for a little one. I use mismatched buttons on the grey version and matching on the red one so the choice is yours.
There are instructions for both a boy and girl version on the pattern and is available in sizes 0-3m, 3-6m and 6-12m.