I am assuming you want a program you load on each PC, if you really want to deactivate the DSL itself you must have a router that supports this and there are a number of very good ones. You are better off using the PC since you cannot always control what wireless network the kids can connect to.
The best software I have seen for the PC side of things is put out by bluecoat. It is a free home version of their commercial software. It is almost impossible to bypass and it plays a dog barking sound out the speakers when the kids try blocked sites. This works much like a virus software where it download lists of sites so it is kept updated. This list of updates is exactly the same feed they use on the commercial license.
This is completely free with no strings attached and no adware or spyware. I guess they just want their name known when people go shopping for $30,000 corporate filter devices.
I'm looking for a parental control software that does these specific things:
1- Controling ADSL connection by activating / deactivating access to internet with password.
2- Blocking adult content.
3- Log all visited website urls
The software must run on Windows XP and Windows 7.