Call Red Top Executive Sedan (not Red Top Taxi) and explain the exact # of people (including ages of the kids) and amount of luggage. You can get anything from a standard sedan (way too small) to a bus (way too large) and they quote in advance, are reliable and it is all handled on a CC, so no messing around on the trip.

You might want to compare it to two Uber XL's, but depending on time of day, you might not get two of those in a given location in Fairfax in the same time-frame. Pricing might not be much different either.

Lastly, ask around your friends & neighbors for a local one-off car service on which they rely. Word-of-mouth people can be very reliable because if they disappoint you, referrals is how they get business.