I have been trying to lock certain channels and have seen that it can be done by using the app button. Looks easy, but doesn't seem to work on my box. I have rolled back my firmware in case it's a firmware issue, but it still doesn't work.
I have a solution that I think will be good enough unless you have some clever little ones who love to play with the settings. It's a bit time consuming, but worth it, and once done, easy for you to temporarily view certain channels should you wish, and set it back for the kids:
Go through every channel and set them as a favourite. Do not set any adult channels as a favourite. When in a sub set, push the green button (SORT) and set it to favourites only. If someone now enters the adult section it will not show any channels to choose. Anyone entering the adult section will think that there are no adult channels available. Unless they push the green button and change the setting, the adult channels will never show.