Here's a neat touch for a sports enthusiast. It does not cost much and will withstand the wear of young boys.

The exact measurements is determined by the bed this is used on.   It is constructed of 2" schedule 40 PVC pipe with standard fittings. The netting will take some searching to locate. You can use an inexpensive volleyball net. It would need to be seamed together.

The approximate size of a goal for a single bed as pictured is 4 ft high by 4 ft wide by 20 inches deep at the base and 12 inches deep at the top. After assembling and gluing the frame, spray paint it with red enamel (or a color you prefer)