The first and foremost compliment I can put forward is that it is a smooth read. That's not easy to do. Descriptions and actions are not over or underwritten. There isn't very much wasted space, save for a couple moments that feel repetitive. There is, however, one thing missing.
Early on, it would be good to have Monica tell Darren a fake story about a monster that lives in the woods, just to give us a little more context as to how she's trying to scare him. She first mentions a monster at the dinner table, which made me think "what is she talking about?" which is never good.
As for the repetitive scenes, Lynn tells Darren twice that there's no monster in the forest, three times if you count when she chastises Monica at the dinner table. We don't need that moment as many times as it happens. I think once is enough.
The story is well paced despite this and no scene goes on for too long. It all builds up to a tense and scary climax that hints at what the monster is without revealing it fully, which makes it that much more effective.
Characters are somewhat flat and don't change, but that's to be expected in what is essentially a tragedy. Having the daughter be the one who is killed is the best choice, seeing how insufferable she is. Leaving the fate of the mother ambiguous helps sell the final scare at the end. The one thing I might suggest is to see if you can make the characters a little more active rather than passive, as that will make them more compelling.
Dialogue needs a little work. Some of it doesn't feel natural, particularly when Monica teases Darren about the monster. The way she talks about it is awkward and not at all the way a teenager talks. Also, cut out whenever someone says "crying wolf". People don't talk like that.
All in all not a half bad horror script. If this does get turned into a feature, there is a larger story you could tell with the mother trying to protect her kids.
