We just booked an S1 with a connecting veranda room for our 10 yr old daughter on an S class ship. No problem with the booking since the rooms are adjoining, and no need to list an adult on her room (booked directly with Celebrity). Celebrity even listed her name on the room alone, and I keep needing to remind them that I am the responsible party for payment!
Her room charges were only a little less than the double occupancy charges (adding a second person to that room, which we may do, will incur only additional taxes). Celebrity confirmed that she can eat in Luminae with us at no extra charge. Technically it is on a space availability basis, but likely not a problem.
FWIW we usually sail with the three of us in an RS on M class ships, and found it more than large enough despite the single bath with no powder room. The two rooms just happened to be less expensive this time!