Well, I disagree with the first statement because everybody loves songs, even 10-year-olds and older! The thing is that these songs must be adapted to their likes, in my opinion. They might think it's something babies do because they had done in the past, but they can see it in a different way if the teacher encourages them and gets involved. So, I think ending a lesson with a song is a good idea, but I try not to abuse of it, so that they can really enjoy it.
About the second statement, I usually start the lesson with a review, but this review is normally about the most essential parts of the unit we are studying. I try to end the lesson with a leniant activity, such a game or an oral activity (not always a song).
Finally, I think it's not good having children in class with nothing to do, so I sometimes encourage them to start doing their English homework. This way, if they have any doubts, they can ask. I sometimes prepare extra exercises, but not always.