"Zijn de kinderen aan het fietsen?"

Translated in a positive way: I would not have had to write this comment if duo also accepted riding a bike. Because where I am from. "Riding a bike" is exactly the same as "biking"

Dont vote this guy down. I know exactly what he meant.

No, it's a question, the equivalent of your question would be "de kinderen zijn aan het fietsen?" Implying you wouldn't expect them to be biking right now.