My kids are a few years behind yours, so I'll be interested to hear what people have to say. I think you're already ahead of the game just knowing that you do need to take some precautions this early on. A not-so-computer-savvy friend of mine had her 5 and 7-year-olds freely using youtube. "They're just watching videos about this video game they like!" She had no idea how many of the 'related video' links on the side of the screen were already venturing into inappropriate territory.
When my son was 8 years old he started watching youtube videos on his ipod touch. They were all related to minecraft in some way or another, but even then there were adds or related videos that we fairly inappropriate for an 8 year old to see. We started monitoring his usage alot closer after that, along with talking to him frankly and openly about what content was ok for him to be watching and what content isn't.
When he is playing online games on the Xbox (plants vs zombies garden warfare is his favorite), he just doesn't wear the headset, and I have all chat muted. He knows he is playing with other actual people, but he doesn't get to verbal interaction from it. I am sure there have been many times when players read my profile, 40year old lifetime gamer, and get frustrated watching me play like an 8 year old with attention disorder.