I listed a father/son duo that got banned for this reason. The father allowed his son to have a regular Niantic ID account connected to Google, but the son was not yet old enough to have a Niantic ID account and should have been playing with a Niantic Kids account. They would both nominate and review from each other’s accounts, and approve each other’s submissions.
Other families have gotten bans, but most times, they are adhering to the guidelines and ToS. The link provided about the family that received bans for review circles has been posted by another, so you may want to check that out.