Android/iPhone: We've long been fans of Sworkit for its randomly generated, equipment-free workouts. Sworkit Kids plays off that theme, but targets the workouts to kids aged 5-14.
Like the regular version, Sworkit Kids pulls together a set of exercises based on the amount of time you have. The kids version makes things a little more fun by working in exercises like hopping on one foot or the tightrope to the mix, but otherwise it's pretty similar to Sworkit. Either way, it's a little more fun to do and has plenty of options to keep things interesting.