Here is SafeWise’s Review.

Smart B’zT: Bluetooth Pick
The B’zT Patch looks like an ordinary patch, but is actually a Bluetooth tracker for kids. It has a low price tag and limited range, making it a good fit for bigger families who keep their kids close.
The patch uses a proximity-based Bluetooth chip that can withstand a turn in the washer and a tumble in the dryer. When your kiddo wanders out of range, you get an alarm that lets you know instantly. The app lets you register up to five kids, and you can track them all at the same time.
And the battery should last up to 10 months—but the lifespan can vary depending on how often it’s worn and the number of washes.
The B’zT Patch uses Bluetooth instead of GPS, so that limits the tracking range. The company claims that it works up to 130 feet, but walls and other obstructions can interfere with that range.
Also, each patch can only be paired to one phone at a time, so if you want to hand off tracking to another caregiver, you need to disconnect your phone and then have them connect using their device.