What question regarding reindeer do kids ask Santa most?
You'd be surprised.
It's not 'what are their names?' - though it comes up often enough for Santa to need to know the answer without having to pause or think:
"Dasher, Dancer, Prancer, Vixen, Comet, Cupid, Donder, Blitzen… and Rudolph!"
Rather than 'who', the most frequently asked question is 'where are they?'
Here's Santa's answer: