We tried "Sworkit Kids" next. I first heard of Sworkit on Shark Tank - they've got an app for adults where you can build customized workouts that you can easily do at home with no additional equipment. Sounds great!
Even better, they've got a kids-focused version, though they do describe it as being for kids 7-14. So I use this app with him and make the key decisions, and then we do the workouts together.
First, you select the type of workout - strength, agility, or flexibility and balance (or you can create a custom workout but I haven't done that yet). You can choose a time (5 minutes and up) and the app puts together 30 seconds of various exercises. again with a short demo (5 seconds) and then 30 seconds to do it -- with the exercise continuing to be shown. Bonus is that this is designed for kids, so you see kids doing the movements.
I really, really like this app. Watching the kid do the exercise while the countdown is happening is key. After every 5 exercises, it gives a 30 second break which is nice. The graphics are clean, the voice guidance is calming and clear, and there's a ton of customization in here to be explored. It's also a free app and ad-free. Highly recommended!