In University I had a practicum supervisor who said "sometimes the sign of a good session is when they are not sticking their tongue out at you." I have never found anything in my clinical practice to be so true. Sometimes an amazing session means a today a child cried for only half the session, or only got up from their chair three times instead of five, or needed two breaks instead of three. To me, these small steps are really huge! While its true sometimes the steps seem tiny, every step forward is an achievement, and positively reinforcing these steps is what motivates a child to keep making them. Rapport also plays an important part and students sometimes can't or don't tell you how much they enjoy themselves. The beautiful thing about music is that it can be a positive reinforcement all by itself.
When I was an intern I had a student who had some pretty large behavioral issues and would remain at the back of the group for the duration largely without actively participating. One day as I was announced it was time to sing my goodbye song, the student burst out "allllllllllllready?!" and that's how I knew it had been a good session. I also learned something very important that day. What looked to me like a non-participatory child was actually a child who was staying calm and actively paying attention for an entire half hour. Once you realize that this likely does not happen in any other aspect of a child's (or their EA's) day, it becomes a pretty big deal.
So, the moral of the story is; its all about your perspective.
And I like to believe that when you genuinely take joy in a child's accomplishments, they can really feel it.