You can't do that with the existing API.
You can use one or more of the following methods to retrieve a known number of children:
limitToFirst(), limitToLast(), startAt(), endAt()
also you can use one of the ordering methods:
orderByChild(), orderByValue() , orderByKey()
Than,
Store them in a data structure (Such as ArrayList) and then Use java.util.Collections.reverse() to reverse the list.
With firebase you should do some of the query logic client side. Not all operations available in SQL are available on Firebase.