Good idea. I can see this being a scene in a horror movie. As it is right now though, there isn't really any build up or anything that creates suspense. It's more of a "shock ending" right now. The set-up, the surprise, the end. There's nothing scary until the creature is born and once that happens, it's over.
Just me, but I always subscribe to what isn't shown to be the scariest. At the end, instead of having the creature be born and show the attack, maybe try creating tension by not having the creature seen. You could have a brief glimpse of the head as it starts to crown (exit the body) so we can see that it's not a human child. Then have Mario close the door and the van begins to violently shake back and forth as blood splatter blackens the windows.
Just some thoughts on what you have written already.
From what I've been told, things like CUT TO and FADE IN/OUT aren't necessary in Spec Scripts. That's something you'd see in production copies.
The second scene heading really shouldn't be the same as the previous scene. I would suggest changing that to EXT. VAN - NIGHT
Also, remember to proofread. Shouldn't Laura be saying "We can't just leave her out here" ?
Tim and Laura follow them. Tim grabs a hold of Laura.
What can we do?
I just leave her out here. Look how
young she is.
Also be careful of incomplete sentences. They tend to jolt the reader out of the experience. I saw things similar to this a few times.
Laura drops down on the floor beside her. Stroke Luna’s hair
and holds her hand. Tries to be comforting.
"Strokes Luna's hair...." should be like "She strokes Luna's hair while holding her hand."
"Tries to be comforting" should be conveyed through action/dialogue.
Also, Characters (in general) shouldn't have similar names because it can confuse the reader. Laura and Luna both start and end with the same letter. They not only look similar, but sound similar due to the beginning/ending sounds of the name.
Like Bert said, I'd take a stab at writing this again and focus on creating the atmosphere and tension. That way when the scare happens, it has the intended effect.