A simple family drama with a horror twist of an ending. The script was a brisk read and it has good structural bones with believable dialogue, particularly the arguments of the parents. Also, your logline is perfect for the script.
Though the dialogue is well done, there does seem to be a bit too much of it. Too much unnecessary dialogue can slow down the pacing of your script. You should try to find areas in the script where you can lessen the dialogue a little and, more importantly, where you can show rather than tell. Like with the rat problem, which we hear a lot about, but never actually see any evidence of.
The twist of having Brittany poison the cookies is solid, but the groundwork for the twist needs to be tweaked for it to really pack the punch that you want. First, the cookies are being baked for a bake sale, which makes the targeting of the mother not really make sense if the cookies are ultimately for Brittany's classmates. This can easily be fixed by changing the reason behind the baking so the cookies are for the family.
The second issue with the twist is a bit larger - character motivation. I can understand Brittany trying to get rid of "the bad" in her mom earlier in the script, but after the talks with both parents and the kitchen scene where the parents are getting along again, it would seem that Brittany would not feel like she needs to get rid of the "bad". I think a solution to this issue could be to have the parents briefly argue a little more in the kitchen. This would give Brittany motivation to use the poison to “get rid of the bad” as the parents argument would sort of nullify their previous assurances to Brittany. A bonus to this change is it would lend to the red herring of the Mom doing the poisoning.
This last note is small. Andrew probably wouldn't have a sign over the garage for his business, even if that's where he stores his gear. I get the script is trying to convey that he's an exterminator visually, which is a GREAT thing to do. Just maybe have something that makes more sense, like his company van in the driveway or his uniform hanging up in the garage.
I hope you find these notes helpful and good luck with your script!
