android - Preserving the state of ActivityGroup's child activities on screen rotation

I have an ActivityGroup inside a tab of the TabHost, and
that ActivityGroup has multiple child
activites.

When device is rotated the tab
doesn't preserve it's state (I'm aware that on orientation changes activities are
restared).

What is the best practice
to keep the state of the ActivityGroup's child activity that was shown before the screen
rotation?

Note that I may need to keep the
states of every child activity of the ActivityGroup.