While my child was riding in the second row drivers side captains chair of my 2018 Odyssey, she fell asleep and unknowingly moved the shoulder strap portion of the seat belt behind her head/ upper body. I was driving and could not see that the belt had been moved because she was seated behind me and was also covered by a blanket. After the vehicle was parked, she unbuckled the seat belt and attempted to exit the vehicle. As she got out, the seat belt retracted and tightened around her abdomen and torso, trapping her against her seat. The belt became extremely tight and we were unable to free her after 20 minutes of struggling to create slack and slide her out.The pressure from the belt cause visible injuries to her abdomen. Because we could not safely release her, we were forced to cut the seat belt. The damaged seat belt was inspected by a Honda dealership, which recommended replacement of the entire seat assembly, costing $3,000. Honda customer relations was also notified and provided us with a case number. I am submitting this report because a child was trapped by the seat belt during normal vehicle use, requiring the belt to be cut to safely free her.