You can do this easier like this:
for quest in available.get_children():
var quest_obj = quest as YourClass
if quest_obj.activated == true:
You also shouldn't reuse variable names as that can cause weird errors.
However, you'd also need to make sure all the children are of your class type:
if quest is YourClass:
var quest_obj = quest as YourClass
if quest_obj.activated == true:
#