Instead of grounding your son, why not send him and his sister down to the local Democratic headquarters to gather materials about the candidates - both Clinton and Obama. Then tell them to research the candidates' positions on the issue and fianlly prepare a brief point reporting on each candidate and contrasting them?

In other words, don't punish them (and your daughter is guilty as an accesory here), educate them. They have lots to learn about our form of government, the electoral process and about their rights and responsibilities as citizens. I'd recomment they learn now so they can vote responsibly as 18 years olds.

Oh, and they can pick up cool Obama buttons and stuff at Dem HQ.

You could also consider having your son write out why he could never vote for a woman. Maybe change it and see if he can defend never voting for a black man or for a Muslim or for someone as old as McCain or for someone with a Polish accent - whatever. Good Socratic exercise in self-defeating logic, no?