I'm okay is a new app in beta which will soon be released on the App Store. It uses location based technology to allow family members to check in to let each other know they are ok and rewards them for doing so. It is a kind of foursquare for families to ensure that their children are safe. It is, as the site states, "knowing without the nagging".
The site is still in beta so you will need to provide your email to request an invite when they are ready.