Dilemma, I have this vague memory that you and I have smilar bosses from similar threads (so much so in fact taht I admit, I just sneakily checked my boss's diary to see if he had booked anything in for that time on Christmas Eve with anyone in case you are someone on my team! ).
This is entirely unreasonable. However, I would be very clear that it is unreasonable not because of childcare, but because of common human decency. I would definitely flag it in your personal log book as a request that was done as a bullying tactic, and not as being unparent friendly. No one in my team has children AND I work in the kind of environment where those who are here on Christmas Eve will not be leaving particularly early and will most likely be here until at least about 5 (as opposed to normal departure time of 6:30) but even in my world, this would be considered unreasonable.
What is your normal work day? If you are normally at work until 5:30, I'd simply say, "I'm sorry, that time doesn't work for me because on Christmas eve I do very much want to leave exactly on time. Can we please do it atXXX time". Otherwise, I'd suggest sending a note saying that you feel it would be hard for both of you to pay attention so close to the holidays and could you please do it earlier.
On the off chance he's not a complete twunt, I know, it's unlikely - also maybe check he didn't just accidentally send you the wrong date?