The Gang Next Door (1980)

A gang of city children is secretly practicing to enter a musical competition. All of them plunder their parents' supplies in order to build their own stage and amplifiers. Baffled by these unexplained thefts, the parents decide to hold a meeting as their suspicions turn to a group of camper folks living across from their flat. The tables are turned when the residents take a camping holiday of their own and end up being mistrusted by the locals as much as they themselves did their own neighbors. Meanwhile, the children's gang meets a rival group of kids who are planning to enter the same competition.

Unfortunately the movie The Gang Next Door is not yet available on HBO Max.