We have an 05 KFX50 that runs great as long as the wheels are lifted off the ground - plenty of power. But as soon as you put a load on it - our 7 yr old weighs 52 lbs. - it is gutless. It wasn't like this before, so I'm wondering if it could be a clutch or transmission issue. Any ideas??

 

Welcome to the forums.
I'm thinking it could be a plugged pipe.
This model may have a end cap on the exhaust that can be removed and a screen cleaned.

FYI: your KFX is really a Kymco and a fine machine.
This may come in helpful when asking for help.