Sthorzina (2014)

Viorel is 12 years old boy and he lives with his grandfather near the Danube river. He helps his grandfather to fix the figurines of an old merry-go-round. Mirko is the son of the local police officer on the Serbian shore of the Danube. The two kids have a remote controlled small boat which they use to send each other various objects. When Viorel finds a gun in his grandfather's attic their lives will change.

Unfortunately the movie Sthorzina is not yet available on HBO Max.