Lake Tahoe is in Nevada. You can fly into Sacramento and other airports.
It's half in California and half in Nevada, but the closest airport is Reno/Tahoe Intl (RNO). It's absolutely gorgeous in the summer with lots of activities for kids. Plus, while you can stay at hotels there's lots of condos people rent for super cheap. Some 2 bedroom places with full kitchens, dining rooms, living rooms, etc go for $150/night. It really depends on which area you want.