I do tip the dining room staff for everyone on the cruises when we use the dining room (most cruises we just use the buffet). When we don't use the dining room I distribute those tips to the buffet staff who has worked hard to make our cruise special. I'm only talking about paying 1/2 on 3rd and 4th for the room steward. Replacing a couple more towels and making up two additional beds is not nearly as much work as a whole additional cabin with two floors to vacuum, two trash cans to empty, two showers to clean, etc. As for crowded conditions being harder to clean, we keep our room very neat and organized so that the room steward doesn't have to clean up after us personally. Being organized and neat is the only way to survive with 4 in such crowded conditions!
I would suggest to the OP to tip 1/2 for the child to the room steward, but definitely tip the full amount to the dining room staff if the child eats there.