Working in retail sucks over Christmas and he knows this.
In reality you get what, from 6pm on Christmas Eve until 6am Boxing Day? And he is proposing that he has them for a significant portion of that time? No. It's hugely unfair. It was his decision to leave and being brutal he made his bed, he can fester in it.
I would also bet that if you were to agree to him bringing them to you at midday on Christmas Day he won't be on time but an hour or so late "because they were having a great time" equally if you agreed to him picking them up at 12 he would be banging on the door at 10.30 wanting to see the DC.
He doesn't get to call the shots. You do.
I would also tell him when he starts waffling on about the financial stuff to put it in writing so you can forward it to your solicitor. Don't discuss it, just be like a broken record...put it in writing, I will send it to my solicitor.
He's being a twunt.