Whether or not your child will know if Bark is installed on their devices depends on what kind of devices you are monitoring.
When setting up Bark, parents will need their children’s usernames and passwords for social media apps that parents wish to monitor. Parents are also able to monitor their children's iOS text messages without installing anything on the iOS device. After those accounts are connected to Bark, children won’t know that they are being monitored unless you talk to them about Bark.
We always recommend that parents discuss Bark and internet safety with their children, but we understand the need to use Bark without their knowledge.