In general your main thing will be control of their gadgets activities because children usually ignore all explanations and warnings, especially in small age. In my opinion there's no need at all for them to use gadgets in very small age, I mean when they're 3-4 yo, like it happens in some families. I think that this is ok only for a limited time and for watching only cartoons sometimes.
Later when your children become older you can pay more attention to explaining and teaching them how to use gadgets and net surfing wisely. Pay their attention to possible web dangers and risks, like scam, cyberbullying, avoiding of sharing with personal information, photos and so on.
You'll need also to block some content because of their age, there are a lot of parental control software now for this like for example Social Shield, Net Nanny, Refog Keylogger
www.refog.com, Kidlogger, Family Shield, Norton Online Family and so on. All of them help to monitor children gadgets activities and protect them from harmful content for their age (like the one connected with porn, drugs etc.).