My advice would be to book as two separate reservations with one adult and one child and request connecting rooms when available. In our experience the cost is about the same. They cannot guarantee connecting rooms, but will try to provide them when available. We did not know about this on our first
ABD, and were squeezed together in one room at two different hotels while other families had paid the same price and had two rooms. On our next two ABDs (BSM and London/Paris) we booked as two reservations of two. Out of the 4 hotels stayed in over those two trips, in 3 out of 4 we had connecting rooms. In Paris our rooms were adjoining but not connecting.