You cannot do it from the router.. If they are Macs then apple built the parental controls into the OS.
Here is the first article I found.. but you can easily search and find more. The exact OS your kids are using helps.
OS X Mavericks: Set up parental controls
I do not think controls at the Mac is the best idea.. frankly kids work out pretty quickly how to get around things.. and it is the wrong point to control.. so buy a router that has excellent controls.. You can keep using your TC in bridge to this router.
What I recommend is a third party firmware called gargoyle.. just search for that too.. if I post the link, apple delete my post.. for whatever strange reason they have.. you need to buy a suitable router but they are not expensive.. The WNDR3800 which is no longer available new but plenty on ebay.. or Dlink DIR-835 is fairly cheap and excellent for the job.. there are some newer ones as well. Nothing over $100.. and that will allow you full control.