Your options depend on your device OS. If you have iOS - based device you will be able to lock down your phone, you can follow the link to Apple Support here.
Android OS's have more options - you can use a monitoring service to lock, screen, monitor and report on content viewed, accessed or where an attempt was made. Two options are here: Covenant Eyes, Net Nanny.
Please take heed, there is no perfect solution to control the content your children access. These steps help protect your children from accidental discovery, but where there is a will, there is a way. It is paramount that you see this a tool to aid in your parenting and guidance and not a replacement. The internet is a dark place with many bad actors looking to work a way to get to your children as has been witnessed by the recent YouTube Kids stories.
I hope those links help you out!