Is dragon kid covered in some intense magic or mana? MC probably doesn't get affected and Mage Noona is probably strong enough to not be affected. So they can act normal with her.
Dragon kid probably thinks the mage is an actual possibly dangerous stranger, especially when seeing their aura. The old mage probably experienced a lot in her life so her aura could be more complex even if she's a good person... like, a good person that has killed many monsters and others wouldn't be completely monochromatic (so it's harder for a young dragon to determine her alignment.) Or her emotions are more complex as a whole and dragon kid doesn't know what emotions those are yet.
Meanwhile the MC is just as regular as can be with a simple aura with a familiar emotion.
Perhaps similar to him, the kids that dragon kid tried talking to have clean auras but sadly they can't handle a dragon's atmosphere.