select atleast one item validation for ASP.Net CheckBoxList

I have a checkboxlist control inside a usercontrol. In the parent page there are other usercontrols also. so when a person clicks the save button on the parent control , there should be some validation that at least one checkboxlist item is selected. . Better if we could provide error in a popup ...

i tried something like this :

in the parent page but am not able to figure out why it isn't working.. tried with customvalidator also..