If you want everyone to be included, I'd ask your parents to join you at around 5.30pm for a pre-dinner drink, and a catch-up, then have dinner at around 7.00pm when your son arrives home from work. His girlfriend can join you as early as she likes. Seems that due to their working hours, you would only be able to get everyone together for dinner at 7.00pm, as lunchtime wouldn't be possible for your son & his GF.
The only other solution, if your parents are quite old, and don't like to eat so late, then I'd do a late lunch/early dinner with them, maybe around 4.00pm, and they could spend an hour or so with your son, and his girlfriend, when he gets home from work, and you'd just have to plate up a dinner for them to eat at 7.00pm.