There doesn't even have to be a kid involved. Couple travelling together, take the window and aisle. Another pax is less likely to choose that middle seat than they would be likely to choose a window or aisle seat. So the couple is more likely to have an empty seat in their row.

If somebody does take the middle seat, they are usually happy to trade for window or aisle, so no loss to the couple in this event either.

Totally standard for anyone marginally savvy imo. I don't see how a lap child changes anything really.