Best lens for "Santa" photos

I've been roped in to doing the photography at a community event this weekend and the majority of the shots will be Santa with kids sitting on his knee - you know the usual thing. It will be in outdoors with ambient light at night.

Equipment is a K200, AF360FGZ flash and my two favourite "auto" lenses, the DA16-45 and the DA55-300. I'm leaning towards the telephoto as the main lens for the Santa stuff so I can frame in although I expect the 16-45 would be sharper and better suited to the flash.

Can anyone give me the benefit of their experience with this sort of photography?

And no, I will not be buying any other equipment prior to the event so don't bother suggesting!