**Compassion and respect**

Expanding on my earlier comment, and to draw from other answers, we can all agree that Dumbledore and McGonagal are arguably two of the most magically gifted wizards at Hogwarts. Both fully capable of performing magic that would render having to lift a child a simple task. 

However, they are also **teachers**.

They have a responsibility for the safety and health of the students, and would be expected to *care* for one who is ill, injured of unhappy. 

Yes, they could have levitated Colin, but IMO if they knew he was safe to hold, then they absolutely would have carried him because of their duty of care as his teacher and responsible guardian.