make a small part at the same pivot point as your aileron and tag it as your flap.  Animate the flap if  necessary (to use stock animation, it's tagged as flap, otherwise, you'll have to use the pct_flap tags...)

Then make your aileron a child of the flap (in the aileron's part properties, select the flap as the parent part)

I prefere this method rather than deal with "flaperon" tags, etc.