You can call the dining line and see if they can add 2 more to your reservation, but realistically BOG is the hardest res to get at this time so it's not too likely to happen.
If the CM can't modify your reservation you will need to check online multiple times a day to see if a res for 6 comes up. You can also try for 2 at a time very close to your current res. If you find a 2 you can ask when you check in if your reservations can be seated together. CMs will do their best. If your parents don't come be sure to cancel their res by the night before so you're not charged the no show fee. If you get a res for 6 you will not be charged the no show fee for the missing parents as long as at least 1 person shows up for the res.
You can also subscribe to the cancellation thread for your month that is on the Dining Reservations page.
If you have no luck getting a res for all of you as a last option when you check in ask if the 2 extra can be seated. As much as possible CMs will try to accommodate you, but there's always the outside possibility that they can't so be prepared with a back up plan just in case.
