Just did a parent page and child page test on my localhost running Alora 2.8.1
There is no problem with using child page.
Please see the screen-cast of my test.
Did you make any changes to your permalink structure?
Please also check that your child pages are not deleted and the permalink is the same as what you have in your menu. If not you will need to re-setup your menu.
Please also clear any cache plugin’s cache after you make any changes.
You can also try disabling plugins one at a time to find out which plugin is causing this issue.
Please clear any cache plugin’s cache and browser history when you are disabling plugins.