I completely understand where you are coming from but I disagree with adjusting a tip downward because the child is a toddler. I cruised with four of my kids last summer- one cabin. I was hit with a $50 gratuity fee per day. Leaving aside the issue that this is so minimal, on a per person basis, I think that if you look at the situation you will see that kids require every bit as much service as adults do. They might not require wine to be corked or directions to be given, but you will find that you are interacting with the staff much more with the baby than you would be if you were cruising with another adult. Ten bucks per day is not a lot when you consider it "pays" the cabin stewards who will bring extra dry sheets and vacuum cheerios up off the floor every day, the waiters who will bring extra half-cups of milk and plain pasta, the busboys who will clean mashed bananas, toast, apples, bagels, melon, etc off the high chairs and floors and tables, yadda yadda yadda. I could go on here but you get my drift!
Ten bucks per day is not much. The bigger your family is, and the longer your cruise is, the more that will add up. These variables are in your control- but you got to pay to play!