Hi rosita, and welcome to cruise-chat.com!
If you haven't seen it already, click here
for a diagram of the inside cabin layout on Mercury. The upper berth will fold out over one of the beds.
I was on near-sister ship Century several months ago, and got a look at some of the cabins in this category, and they were not much larger than our balcony cabin. Our cabin measured 175 square feet and had a sofabed, and when the sofabed was opened, there wasn't much room to walk around.
How old is your child? One thing we used to do when our son was younger was to bring a thin inflatable mattress for him (the type used under sleeping bags), which would then slide under the bed in the morning. This would take up far less room than a rollaway.