It can be done using Map/Tint by Element. In your scene, the "house" material uses it, but "tree" material is on Item mode.
If you want to randomize complete items, but not elements parts, then it's not possible because internally nested objects are handled as a collapsed mesh. There is not reference to the original items.
Alternatively you would use groups instead nested. In this case, each object is scattered individually, but keeping the group disposition. To do it, just create a group and assign it to Forest selecting by name (picking doesn't work for groups).