Hmm. I can see it.

Pike probably half-heard one of the kids running in and talking to the others about the battle at the gym, Pike himself not actually knowing what's happening aside from the fact that they seemed really excited and determined to get over there, which would catch Desmond's attention, and possibly April's as well. Desmond would certainly be interested in seeing how the Gym Leader battles, and wouldn't mind going over to the Gym and watching the battle before entering one of Pike's classes later on in the day. 

Though... The truth of the matter, once they realize it, will anger Desmond greatly. That I can promise you. Pokemon theft isn't something he takes lightly.