Well, here's the thing....they might be assuming that you CAN afford the trip since you have three kids going to their private school.
Now, if I were to send my 5th grader (10 year old) across the country for a field trip. I would have to know:
WHERE they are staying - DC isn't just some section - it's huge. And some parts are NOT good areas....so are they staying in Maryland or Virginia and taking charter buses into DC every day?
WHO are they staying with - what's the chaperone situation? How many kids to a room? What adult will be staying in the room?
WHAT will they be doing for the field trip. There's soooo much to do here it's not even funny...and we're NOT even talking JUST DC - we're talking the battlefields around here - Gettysburg, Winchester, Antietam, Fredricksburg, Manassass.... that's just a few....
Does not going affect a grade? If not going means my child might lose a full grade or credit - I need to know these things...
Would I send all three? Only if I could afford it.
Would I go with them? Only if I could afford it.
Need to make a choice/decision about what you NEED and what you WANT. Family vacations are great. Education is important too...
Other things to consider....is my child MATURE enough to be gone for one week without me? If the answer is NO. Then the decision is already made and no reason to ponder other issues.
I would seriously look into the budget and see where things can be "fixed" to allow things to happen. Maybe instead of a week vacation during Spring Break - you can do a long weekend over Memorial Day. Vacations do NOT have to extravagant nor do they have to be far away....living in Los Angeles - if that's really where you live - then you have Santa Barbara north of you - great place to stay!!! You have San Diego south of you...much to do there as well....you have to look at more than just ONE thing...and ONE idea...what you have done in the past...do you always have to do it that way? No. So think outside the box in regards to family trips.
If your daughter is mature enough to follow the rules, stick with the group and behave - and be without me for one week - then I would send her..