I would rent a car, but only if there is a rental agency located in or near your hotel. You can usually find a mid-size or less than $30/day, so even with gas and parking, it's cheaper for a family of four to drive than take a bus.
If you took a taxi in rush hour traffic, I wouldn't be surprised if the cost was more than $75 each way. The 101 runs right along downtown and if you're planning on being at US for park opening on a weekday, I would budget 90 minutes to drive there, park, and be at the gate with tickets in hand.