Having lived in SF it's about a 1 1/2 to 2 hour drive each way to Napa. It can be done on a day trip but because you'll probably be drinking at the wineries I'd recommend staying a night at a B&B. Also, since teens can't drink it may be a bit boring for them. What might be better is simply driving through Napa on your way north simply to see the scenery and picking just one or two wineries to visit. Unless you're doing the tastings one winery tour is enough, two and you will have a great feel for it. Three + is for aficionados.