I remember this from when I was a kid.. it may very well have been out of the early 90s, but I just can't remember it well enough.

The whole theme was monsters that lived under children's beds, the underside of a bed was like a door to a monster world. There was one really cool monster who could only come out at night and was friends with all the kids.

At one point the kids and their friend monster guy (donning a leather jacket and hornes on his forehead) pissed in a bullies apple juice jar.

And at the end they fought some kind of boss who was a kid with the back of his skull removed, revealing his brains. One of the kids was a girl with a science project that let her shine light all over the bad monsters.

And at the very end they ended up on a beach somewhere.

This is all I can remember. Just surreal memories in the back of my mind of watching this movie with my brother/sister. Someone has to help me out on this, I know someone here knows this movie.

  Sounds like "Little Monsters" released in 1989. Maurice(Howie Mandel) as the monster, Brian Stevenson(Fred Savage) as the boy. Also appearing Eric Stevenson(Ben Savage), Keirsten(Amber Barretto), the head monster known only as "Boy"(Frank Whaley).

Yup, has to be Little Monsters.  Cute movie.  My kid's favorite when she was little.

Oh my god. Thank you all.

It is Little Monsters.