code: func (bucket *kbucket) RandomChildID() string {
  	prefixLen := bucket.prefix.Size / 8
  
  	return strings.Join([]string{
  		bucket.prefix.RawString()[:prefixLen],
  		randomString(20 - prefixLen),
  	}, "")
  }
docs: // RandomChildID returns a random id that has the same prefix with bucket.