George Mason .. his extreme unhappiness with slavery, but his reluctance to ruin his children who would have been almost paupers if he had freed his slaves.

Interestingly, Jefferson initially drafted the Declaration of Independence with a scathing indictment of slavery. This was deleted at the insistence of Georgia and South Carolina, it being crucial to obtain unanimity among the colonies. (Politics is that art of the possible.) But note that 11 of the 13 colonies accepted this language -- a position far ahead of their time:

he [King George III] has waged cruel war against human nature itself, violating its most sacred rights of life and liberty in the persons of a distant people who never offended him, captivating and carrying them into slavery in another hemisphere, and to incur miserable death in their transportation thither. This piratical warfare, the opprobrium of infidel powers, is the warfare of the Christian king of Great Britain, determined to keep open a market where men should be bought and sold, he has prostituted his negative for suppressing every legislative attempt to prohibit or to restrain this execrable commerce; and this this assemblage of horrors might want no fact of distinguished die (emphasis added)