There's a way to frame it. At the end of a band performance, the audience (even if it is just 3-4 people) is encouraged to clap, even if the performance sucked. Usually, they do clap. This is enough of a positive reinforcement.
When the whole thing is over, before packing up, have a 15 minute meeting in which all you do is say,
"Great performance guys. You did a,b,c really well. Jeff, I saw you listened to my instruction. Hi five! Stacy, you did a good job. There were some mistakes in your chord transitions, let's talk about that so you can do it better next time"
Kids should grow up knowing they are doing well but aren't perfect yet. Let's keep improving until we get there.