Welcome to Android Central! If the Setup Wizard is appearing because your child somehow caused a factory reset, then you've lost your data anyway. But if he just went to the App Drawer and opened the Setup Wizard, then you shouldn't have to worry about losing data. If you can't exit out of the Setup Wizard by tapping Home or Back, then try forcing a shutdown by pressing and holding Power for 15-20 seconds (or, if you have a Samsung device, Power and Vol Down together for 15-20 seconds). Then Power back on normally. If you get the Setup Wizard again, that means the phone had been factory reset.