If they're old enough to surf on their own, they're old enough to handle it on their own.
It is - to a degree - your call if they are old enough to do so, but countermeasures to keep the "bad internet" away from your children, if you are geek enough to allow them access, is a bit of an oxymoron.
Hint: If they want to see porn and/or Isis set someone on fire, they will do so. If not at home then at/with their friends. Trying to prevent this is being silly. Once I trusted my daughter to handle her own Ubuntu Netbook I also trusted her to handle the web. ... I did curb her webtime though, it can get out of hand. ... But she uses the web and her smartphone as an extension for her social life, not as a substitute. She's actually more on the go than I am, and unwinds not surfing but streaming american teenie serials to improve her english (currenty the 100 is hip). Not the worst thing to do, imho. Her homework gets done and she's due for her a-levels, so who am I to complain?
I had a discussion a few years back with a mom of one of her very close friends. She too was worried that the new laptop would enable them to watch porn and get a false impression about sexuality. I basically said the same thing that I wrote above and bit my lip about her habit of changing boyfriends every odd month - something way more likely of determining her daughters POV on relationships and sexuality.
Ask them to learn something productive with them - my daughter eventually decided to do a little image editing and I got her a neat colorful book on Gimp of which she duefully did some excersises and learned a little about files, photography and image manipulation. Good thing for a teenage girl exposed to a cosmetics/fashion industry in constant overdrive. She didn't want to learn programming though. ... I'll survive that I guess.
Tell them about Facebook, Whatsapp, data mining, automated 24/7 surveilance, scams, rapists, shady friends, online mobbing (both sides of it!), etc.. Give them fake accounts and tell them to never use their real name and adress and to be suspicious of the web in general - including mainstream news.
Be a good father, take care of your kids and make a reasonable judgement as to when they're ready to have their own computer.
Do the basics to keep them out of harms way (hint: porn is way, way down on that list) and make sure they've understood what you're talking about and have no fear of coming to you whenever they're insecure about something internet related. Let the rest take its course. ... That's parenting 101 for you.
My 2 cents.