Is it normal for a 20-month-old's back to crack all the time?
A pediatric orthopedist I know calls this painless popping and snapping "joint music." The ankles and knees are the most common areas for this to occur, but as long as your daughter's back cracks are painless there is no cause for oncern.
These pops and snaps occur in children because some of their joints are more flexible and more loosely joined than ours are. If it ever seems to hurt when a joint pops or snaps, on the other hand, contact your daughter's pediatrician.