You should take the time to sit down with your child to show them how to capture screenshots on any of the devices that they might be using. Encourage them to capture evidence as soon as they see anything that is harmful, inappropriate or upsetting. Once they have the evidence, encourage them to speak to you or another trusted adult.
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parental control how-to guides to get up to speed on how to set the right level of
protection to give them a fun and safe experience.