To set up Footsteps2Brilliance so that your child(ren) cannot exit the app until you are ready, use the built-in features on your mobile device.
For an iOS device, you can set up Guided Access.
Learn how to set this up: https://support.apple.com/en-us/HT202612
For an Android Device, use Screen Pinning.
Learn how to set this up: https://support.google.com/nexus/answer/6118421?hl=en