If the bike is stored with the wheels off the ground, then giving them a spin every couple of weeks should be sufficient. That's how I keep mine fine over winter.
For no maintenance by daughter/others, then yeah the best option is probably to take the tires off, clean up the sealant and set them up fresh before use. But that way you risk rim damage from having bare rims or flat tires.
You could always just put tubes back in since it's a spare bike that's not getting used much, and save all of the hassle.