For windows there are the parental controll settings. Android also has this for some time, so you do not need a latest version. Iphones most probably also have this settings too.
Just visit the settings of their devices and configure it there. The settings might be slightly different, alt least for iphone.
This might be slightly out of scope for this stack site, but my advise is to make web wrapper type of app. You can use ionic or react native.
With ionic, you more or less make a website with HTML,JS and CSS, which is then bundled in a webview/browser wrapper that appears as an app (supports both android and ios) to the user. Although 1 week is quite short to ...