Most mobile devices offer security settings that you can use to make your phone or tablet safer for child use. Some tablets are designed specifically for children, such as LeapPad. Other tablets offer services for children, such as Kindle fire for kids, which you can sign up to. 

Listed below are links to some websites that may advise you on this.

Please be aware, however, that no security setting can replace child supervision and open conversation with your child. For more information, visit Internet matters or Think u Know.