I also have a similar problem. After upgrading from 5.6.1 to 6.0.0.
Parent page opens normally, and the child pages can not be opened. Show a 404 error.
Then upgraded to 6.0.1 and still not open subpages ((

How can I fix this error?

Dorofei Nikiforov, I figured out a work around. It's a pain in the butt and not very optimal from an SEO standpoint, but it will get your pages up. Here's what I did. FIrst, you need create the exact same directory structure under another parent page. For me, my page was dealer services so I created a page called dealer services pages with all of the same child pages. Then, you need to move all of your sub pages somewhere outside of their parent. You can then see them. Get all of the content off of them and put the content in the corresponding child pages. Now, move your original child pages back under their parent, this will break the links again. Go to each page from the pages module and change the page to redirect to the new pages you created. Make sure those new pages aren't included in the menu. This way, you have the menu structure of old, but those pages redirect to the new child pages and they work. I guess if you don't need the exact same site structure, you could just create a new parent and move all the pages under it, that works too. I just need to have the exact same links.