Can you browse to https://nordvpn.com when on your home network? Yes, then Web Safe is on otherwise post back here to have the issue flagged to the forum team.
If the child's device can be configured to stop its settings from being changed and apps being installed (i,e, parental controls) then making sure its network connections are configured to use the DNS settings suggested by legacy1 will likely stop a majority of inappropriate content but not all when it is being sought out.
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