Find the Menu button on the TV and select the Child Lock in the list of Options (use the onscreen Up/Down arrows. Chet child lock or or off. Note, to do this you have to add a password. I am sure the TV has all the instructions, either on the TV screen or in a Manual.

It might be impossible to release the child lock, without its remote control. If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/
Unlock procedure to many popular brand TVs are given.