When the baby is already born and you want to make a personal gift, Liten Socks may be it.

The faux cable pattern is explained and done directly on normal needles, it makes the socks quite elastic.

I would use 210m/50g yarn on 2,0mm needles for a 6-month old, and 175m/50g yarn on 2,5mm needles for a 12 month old baby.
Half a skein is plenty for a pair.