Am I being unreasonable?

[Naftafia]

You are not being unreasonable at all. Taking all the kids out for the school run takes a lot of work and some preparation, the kids can't be expected to just up and go without a warning.... and be cooperative

I completely agree that you should give him a time limit for drop off, as you are considering. I would write him a notice type letter clearly noting the specifics and try to add some info as to why this impacts you and all the kids, so he could hopefully understand that you are not simply trying to be difficult.

Good luck, I hope it goes well