There a great article from the Good Men Project on how you can block porn on kid's devices. 

There are 3 main ways to prevent pornography on children’s devices:

• Hardware
• Software
• Child-safe devices

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Hardware to Block Pornography on Child’s Devices

A WiFi router is the gate to all Internet traffic that enters your home. Most modern routers have at least some parental controls — either included or offered as a premium feature via a monthly subscription. Parental controls routers like Gryphon or Clean Router are built specifically with children’s safety in mind.

Read the full articel below... 

How to Block Pornography on Kids’ Devices - The Good Men Project