A teenage prankster must convince his father (a cynical cop) that their elderly neighbour is a psycopath before she kills again.

I'm not sure the word "prankster" is the best choice here.

I could see myself watching this. I'm assuming it has a bit of humor to it? I can picture this old lady on a walker, and she has everyone fooled except for the teenager, and of course no one believes him because she is just a "fragile old lady".

Sounds interesting. Here are a few concerns
1 Isn't this film very similar to 'Disturbia"?(starring Shia Lebeouf).
2 Why is the cop cynical?Why is the teenager a 'prankster' or rather what does that have to do with the plot?
3 Why is this elderly neighbor murdering people?
4 How does he figure out that she is a murderer?
I'm sure you have already thought of all these things. I like it though. Sounds cool.