Everyone knows, probably, that kids can get a little smelly.
The really young ones do a lot of things that cause many odors. They may not have body odor yet, but other sticky and smelly messes can occur. Especially if some food, some jelly or peanut butter, etc. winds up getting stuck in the air conditioner and/or the ventilation fan. Kids could probably find a way to climb all the way into the ductwork to serve up a shmear if left to their own devices. Every single activity turns into a finger-painting exhibition. Sticky stuff covers the air conditioner, the furnace, the vents, and even the HVAC provider is not safe. The HVAC technicians would definitely get a generous amount of goo on them without any prodding or encouraging from the parents. And when small children use the restroom the smell tends to be worse. They might be potty trained, hopefully, but for some reason even when they pass gas, the bathroom exhaust fan will not be enough. In fact, if you have kids it might be time to update your HVAC system and all of the vents and fans that go with it. Depending on the number of small children in home, this number will also tell the HVAC business how state-of-the-art their equipment needs to be. For example, the furnace will need a bullet-proof door covering it. The air conditioner control panel needs to be on the ceiling with a lockable door covering it, and all of the vents for various heating and cooling purposes need to be in obscure, hard to reach places, which, of course defeats the purpose of a whole ventilation system. And, why not, every room could use an exhaust fan to whisk away the odorous particles before they become stuck on permanently.