Transform sorting order

Hello!
2020 is very soon so I decided to check the alpha by coding a simple pvp card game. In that game player have 6 cards on hand and I implemeted this via transform order. But when I place my player's hand prefab into scene, the transform order breaks and there's no way to chage it.

In the scene hierachy looks like this (this is obviously not how I designed this hierarchy initially):

Code (CSharp):

1. Player (instance of prefab "player")
2.   Hand (instance of prefab "hand")
3.     Card_1 (instance of prefab "card")
4.     Card_4 (instance of prefab "card")
5.     Card_5 (instance of prefab "card")
6.     Card_3 (instance of prefab "card")
7.     Card_2 (instance of prefab "card")
8.     Card_6 (instance of prefab "card")
9.    

If I try to reorder them as they should be, Unity (expected behaviour) says "Can't restructure prefab instance". But when I click "Open Prefab", I see same broken order and I can easily fix it. But after switching out of prefab mode, the structure gets broken again... I have Auto Save in prefab mode checked, if it matters.

I don't know, how to fix that? Thanks for any help or advice!