"I never do songs with my group of 10-year-olds. They think singing songs is for babies."
I am afraid I must disagree with this statement. These 10-year-olds might be reacting this way because the selected song IS for "babies". Times change, and even children would rather sing popular songs rather than songs supposedly designed for them.
"I always start the lesson with a review of the previous lesson. And I always end the lesson with a song."
I see eye to eye with this statement. Given that young learners have to get used to routines, starting with a review of the previous lesson (logically connected) and ending the lesson with a song is a good way to start them on a routine.
"I prepare extra exercises for the children who finish early. It's not good if they have nothing to do."
I believe this might be a double-edged weapon. On the one hand children might well not notice this extra effort that is being asked to them by giving them additional activities, which is desirable. On the other hand, if they take it as an offense, they might end up being either disruptive, complaining, or simply not telling you that they have finished working and purposedly stall. It comes down on what works best with your group.