A 13-year-old boy convicted of hitting a motorcyclist on the head with a glass bottle after the rider was deliberately knocked off his motorcycle with a rope has been spared more time in detention.

The boy, who can't be named, wasn't involved in tying the rope across the road, but did strike 19-year-old Lawson Mills outside the Perth train station on Wellington Street in December after he was flung from his motorbike.

Lawson Mills was lucky to escape with minor injuries after being knocked off his motorbike by a rope strung across the street and then glassed on the head.

The child, who was 12 at the time and has already spent about three months behind bars, avoided more time in custody when he was sentenced in the Perth Children's Court on Wednesday for unlawful wounding.

He was granted a juvenile conditional release order for 10 months with an attendance and supervision component.