I don't frequent Whistler that often (and usually in the summer at that) so I'm sure there's others who can provide more detailed info but here are some thoughts to get the ball rolling...

Personally, I wouldn't use "twisty" mountain roads to describe the Sea to Sky Highway but there are stretches that road is cut into & around hills. You shouldn't really need a car in town as you can just use the local buses to get around so I'd recommend just taking a shuttle bus to get to Whistler if possible.
[Mini-rant: If you do drive, take your time. The highway is notorious for accidents because people drive too fast/aggressively for the conditions (wet, dark, curves, tired from a day a Whistler, etc),

For family friendly & economical dining, I'd recommend some chains: Earl's, Milestones, Old Spaghetti Factory. You can google them and check out their menus. Take a look at this site for other ideas http://www.dinehere.ca/ but take some of the reviews with a grain of salt.
We usually pick up groceries at a supermarket chain called Marketplace IGA.