MechaTarrasque
Hero
I have to admit, I would have judged this the other way, but RAI according to Jeremy Crowford (https://www.sageadvice.eu/2018/02/07/can-you-cast-dragon-breath-on-your-familiar/) is that:
"Dragon's breath is cast on a willing creature. Your familiar is a creature that can be willing. Find familiar prevents a familiar from attacking, but dragon's breath involves no attack."
It seems like that makes the spell a pretty good deal for the wizard, and a slap to the poor chainlock and the OG Beast Master, both of whom have to give up his/her attacks, while it seems like the wizard just has to cast a spell once, and not take a hit until the fight is over.
"Dragon's breath is cast on a willing creature. Your familiar is a creature that can be willing. Find familiar prevents a familiar from attacking, but dragon's breath involves no attack."
It seems like that makes the spell a pretty good deal for the wizard, and a slap to the poor chainlock and the OG Beast Master, both of whom have to give up his/her attacks, while it seems like the wizard just has to cast a spell once, and not take a hit until the fight is over.