Computers should always be in full view of the entire family!!!! It should be placed in conspicuous areas like the kitchen or family room so everyone can see what you are doing. YOU meaning everyone in the family children and parents alike. There is nothing that anyone has to do on the computer that should not be seen by other members of the family who walk into the room. That said, they of course should not be looking over your shoulder if you are doing your banking, confidential work, etc.
Just knowing that anyone can peak and see that you are “on the wrong page” will help fight the yetzer horah. There is also a way of tracking the history on the computer to see where someone has been. So that is another way to help the family fight the yetzer horah. Starting out this way and putting up filters, using some suggestions from others above, helps the family start off on the right foot and avoid problems.
With all that said keep in mind that boys will be boys and if they want to see and know they will find a way just like your generation did. There are still magazines in stores and there are still movie houses and there are still books in libraries. They are just like you and your friends were at that age. There were many a yeshiva bochur that found their way into the very, very asur theater near the mir yeshiva many moons ago.
So the best way to keep your child from going that direction is to be upfront and honest with them. Have the proper line of communication, speak to them about what is happening with them and what their changes are all about. Explain to them about hormones etc. Remind them that although Hashem is preparing them for the future and all this is good and natural, there is going to be a tough battle between the yetzer horah and the yetzer tov and they will have to work hard to win and control the desire to give in.