I had the same issue with DS.
What i did was went with mum to get them done. Go early when there is no one else around so makes it a quick process too. DS didnt see Santa the whole time... didnt tell him what we were doing at all.. was sitting in the stroller facing the other way. Mum got DD out and get her to Santa to hold. DS still oblivious. I picked him up with his back towards santa and walked over and sat down with DS on my lap. Didnt say anything to him, didnt tell him what we were doing. I had to be in the photo and that was fine but he didnt see santa until later he turned around. He was ok as he was on my lap and the ladies were great in getting both kids to smile.
So maybe you being in the photo might be an idea.. dont tell them whats happening at all and act normal as if nothings going on?

Best of luck.