Q: How do i stop kids using taskbar to close running apps (time limit)

Since there is no way in iOS to limit time usage of an ipad we installed "time limit" which uses a never-ending popup when the time limit is reached. This is the only app/feature we found that works.

By setting restrictions we can stop my son deleting the app, but clever son realized he can stop the app running by double clicking the home button and swiping the app upwards to close it.

Is there any way to stop users accessing the taskbar via the home button, searched and searched but no luck.

Already wrote to the app developer but no response.

Any help appreciated.