Ray’s review published on Letterboxd:
This is probably as good as a "killer kids" movie can ever hope to be. Lean and mean, Bloody Birthday never drags, pausing only briefly to engage in a little babysitter gaslighting. Without worrying about "whodunit," the film spends most of its time effectively building an environment where murder can happen at any given moment in a variety of clever ways. This allows for several amusing close calls that don't undercut the tension. There are a few subtle bits of dialogue that add a bit of depth to the characterizations, but it is just a slasher variant at the end of the day.