You, as generous parent, have given your child an iPod touch. Problem is, they spend all their time on the thing texting their friends when they could so more important things like their homework. You could, of course, just pull the thing out of their hands, but if you connect to the Internet via an AirPort Base Station try this instead.
First, grab hold of their device and go to Settings > General > About. Scroll down and look for the Wi-Fi Address entry. This is the hardware address (or MAC address) that's unique to that device. Write it down.
Now launch AirPort Utility, select the AirPort Base Station that's connected to your broadband modem (in the video I misspoke and said "router," but that's incorrect as the Base Station is the router in this case). Click Edit. Click on the Network tab and then enable the Enable Access Control option. Now click the Timed Access Control button. In the sheet that appears click on the Plus button to add a wireless client. Give your kid's iPod a name and then enter its MAC address in this field. Now configure the pop-up menus for those days and times wireless access is allowed--so maybe for an hour or two after they come home from school during the week and a few hours on the weekend.
When you have it configured to your liking, click on Save. Then click Update. Now, should your child attempt to access the wireless network outside of allowed hours they'll find their way blocked.