1. Try to make any reasonable agreement with child about brushing hair daily. Like before going to bed.
2. You can also try to remark that princesses don't have dreadlocks.
3. If immediate action is required, dump half a bottle of conditioner on child head and set in front of any electronic screen that renders child motionless. Then detangle for up to an hour.
4. If away from home, go to a hair salon. If child is reluctant, go first and have a shampoo and a cut and proclaim loudly how much fun it is.