the find my phone app had my child at a house 4 blocks away from where she said she was. Can the app be that inaccurate?

When you view a location in Find My it will show a circle with the most likely location at the center. But it can be anywhere within that circle. There is one possible but unlikely way that the location can be off. It uses Wi-Fi to fine tune the GPS location, and also when the phone is indoors. If the Wi-Fi router was recently (within a few months) moved to a different location 4 blocks away then that might result in the condition you are seeing.

Yes, it is quite accurate. It can even locate you child in the next block too. The technology is amazing and very useful.