If someone sits in a seat, they must pay for that seat--period. Asleep, awake, "baby," adult, whatever. Seat = pay. There is nothing "offensive" about it.

Also, I don't considering 10 PM to be that late for a small child to be out, but I definitely consider past midnight, when the movie would have actually ended, to be too late. It's a lucky thing you paid for a seat for the child to sleep in, lest she fuss and whine throughout the whole movie. (This is, of course, assuming that we are talking about 10 PM, not a 10 AM naptime, which is not mentioned and counterintuitive.)