Zip Wire Pulley.

thingiverse

This zip wire pulley was tested by person with 100kg and it worked fine, so it should be safe for kids. (keep safe height anyway.) It's made to fit about 12mm rope. Pulley is made to fit ball bearing with internal diameter 15mm so you can use aluminum tube with 15mm diameter for fixing ball bearing and hand grip. Also use stand M6 bolts to join pulley housing. When mounting hand grip use super glue to keep a rod in place. When printing select almost full fill to achieve required strength.