Right now you're paying a single traveler surcharge. It's not the full rate for two occupants, but close to it.
The child rates are based on their ages and if they're the 3rd/4th, etc. occupant in a room. The category doesn't matter.
If you were to move a child into your room, it would actually cost them more as he/she'd be considered the second occupant at full fare. However, it may be a bit of a savings if you compare her current rate, and what the increase will be to you if added to your room.
I know it's confusing. I suggest getting the child fare and what your rate changes to by adding a child to your reservation and seeing if the difference ends up being less.