database.child("usuario").addValueEventListener(new ValueEventListener() {
    public void onDataChange(DataSnapshot dataSnapshot) {
         for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
             for (DataSnapshot messageSnapshot: snapshot.child("mensagem").getChildren()) {
                 listView.add(messageSnapshot.child("textoMensagem").getValue().toString());
             }
         }
    }

    ...

如您所见，我添加了一个ValueEventListener来检索"usuario"的所有子节点，然后循环浏览其子节点以获取您的消息.

As you can see I add a ValueEventListener to retrieve all children nodes of "usuario" and then loop through its children to get to your messages.

提示:考虑(至少)以英语发布您的代码/数据库结构等，这可以使您阅读问题变得更加容易.