Remember to put each child on their own Limited/Standard Windows account. Then use the Parental Controls (or third-party software if using XP) to limit computer usage. If you’d like to filter the same types of website among on your computers and users, then consider OpenDNS to do the web filtering.
For your child’s mobile phone, first consider services offered by your service provider, and then look into using apps for more controls.
For social networking, be sure to review their privacy settings and friends, create a profile too on the same networks, and think about using a monitoring service.
Keep in mind there are always ways around parental controls and filtering. Most solutions aren’t necessarily weak but there’s only so much they can block. A tech savvy kid could, for example, connect to a neighbor’s Wi-Fi to bypass OpenDNS filtering, use a proxy or VPN program to bypass filtering, or insert a Linux Live CD into his or her computer to bypass program and usage restrictions. Plus when they’re out and about they could use someone else’s PC or mobile phone.
So you still need to keep an eye on your children. Start by making sure PCs are used in an open area like a dinning or family room, not their bedroom. Also discuss with them about not sharing their address or personal information, and not visiting people they meet online.