I will probably attend this event, and would be happy to help with ground transportation for juniors. Vlad will probably have suggestions, and he may suggest something other than what I describe below. I defer to him on all that.
For hotels, I would recommend the Marriott Renaissance Club Sport in Walnut Creek. I stayed there earlier this year, and I think it would be great for juniors. It is centrally located to the race venues, and is a nice, newer hotel. It has a lap pool, an enormous fitness facility, several indoor basketball courts, several racquetball and squash courts, and a nice lobby with TVs tuned to sporting events with lots of couches and plush chairs. Lots to do, without having to go anywhere. Rates are $132 per room per night, for up to four persons in a room.
https://www.marriott.com/hotels/travel/oakbr-renai...
A junior who is old enough to compete at JWOC is probably old enough to take the BART. But that's a decision for each family. BART is probably easiest from Oakland airport, but is also doable from SFO. I have not purchased airline tickets, but will probably arrive on Thursday night sometime between 7 and 9. For those arriving on Thursday night, I could probably pick up juniors at the Walnut Creek BART station, and take them to the Marriott. And then provide ground transport from the Marriott to the races, at least to the capacity of the vehicle I rent.