Kid coughing doesn't mean infected balloon is still there, it means the kid needs to recover, it allows you to know that's there's at least one balloon infected, whoever, the kid needs to recover after that, so we don't abuse with this mech (like you tried)
The signs are the next : swap color (a sneaky one), a face on the balloon, a laughing (can be heard very far away), wiggle balloon.
I added a few more obvious (I hope), but the strategy is to observe the balloon a good couple of seconds (careful, some are sneaked behind the door too)
Thanks for playing