Can't move child in prefab instance but...

Hey all. I have a prefab instance like this:

PrefabInstance
- Child1
- Child2
- ChildThatShouldBeAtTheTop

I try to move the 3rd child so that it's the first but it of course tells me I cannot do that in the prefab instance. I then edit the prefab but it looks like this already:

PrefabInstance
- ChildThatShouldBeAtTheTop
- Child1
- Child2

It seems the order in the prefab and instance are different and I can't do anything about it. How do I move the children in the instance in this situation?