The bow is single crochet into rectangle, folded and sew closed. For the older daughter, it is 20 sc chained, about 5 cm high (went by pattern for hers) and the middle piece is 5 sc, until it could tightly wrap around the bow (again, by pattern). I just cut the measurement into 2/3 the size for my youngest.
The headband was by me, I measured their head and did one chain by their measurements, and crocheted 6 sc then branched off to another sc chain dropping about 2-5 sc and crocheting 6 sc together, again in order to allow tying.
I drop 2-5 sc because I want the band to be secured well, not falling off or cutting into their heads. It allows a loosly secured band and a tight one, keeping them in place.