Help to restrict guest user lollipop

[vihuarro.1]

Request help to restrict access to the guest user in a Samsung Tab S2 (8") with lollipop. I tried to remove it and add password but when changing user and back gives you the option to start new guest session, which erases everything. The issue for me is that the tablet is my 9 year old son and has taken to listen to music on Youtube, so I want to prevent it from being exposed to inappropriate content. I have a user created for him with two programs for parental control and as I have restricted youtube, so he goes to the guest user. Even as you install the youtube kids, but still it goes to guest user. How do you see this? Do you think there is any solution? Samsung certainly tells me that for the moment they have not developed an alternative and although I made the request, they will scale and have no time to respond.