First, the iOS restrictions are quite broad, and don't get nearly as specific as you need them to get. These are the restrictions that are built into the device and included with the Operating System. So, understandably they are essentially useless to you.
Since you are dealing with iPod Touch devices, they can only get online through a local wireless connection. So right there, you can limit SOME content (domain whitelist or blacklist depending on your router) but then when they are using another wireless connection they are once again accessing the internet wide open.
There are companies like Mobicip that have developed a quasi-proxy based solution to web content filtering, but you need to pay them for the service, disable access to Safari/YouTube/The App Store and install their "safe" browser.
As it stands, no existing solution will allow you to limit who they add to their list of Skype friends. You can (through skype) limit who you GET calls from, meaning you only get calls from friends, but you can't create a true Whitelist which completely blocks all potential non-listed contacts. Your kids could simply add another friend, whomever that friend might be.
There is no control for disabling game access after a set amount of time. There isn't even a control to disable game access. You can limit or kill MULTI-Player game access... which would kill a game if it was only multi-player, but no way to install software that will shut down a game after an hour of play.
You can filter music through iTunes by enabling the rating restrictions, but that won't affect music they get off the web.
Overall, you are kind of in a bind. The iPod Touch wasn't designed to be a babysitter. You still have to be the parent. You have to make sure they are using it responsibly, and take it away from them when you don't want them doing things with it. So... the only real Parental Control for what you list that you want to do is YOU.