Is it a 3rd party app? If so, it may be being closed due to Battery Optimization, and you can select to never put the app to sleep in that section of your phone's settings (or by long-pressing the app's icon, going to App Info, Battery, and selecting the option there). However, that doesn't address the fact that if it's a Parental Control app, it should NOT give access to a kid's account to close it or manage it without the proper credentials (and that's an app issue, not a phone one).