Given that the difference in price between an economy, compact, and midsize (when making your initial booking) is usually just a few dollars a day, this is why I always book at least a midsize. You never know what's gonna happen.
But of course, you cannot go back and change the past. So, what to do in this case?
You'll have to negotiate with the rental agent for an upgrade. To get the best price, you have to appear not very needy. If the salesperson knows you really want and need the bigger car, he'll charge you a lot more than if you have a choice in the matter.
Here's what I'd do if I were in your shoes:
- Have your other three family members stay behind and out of sight. Don't let the rental agent know you'll be traveling with more than just the two of you.
- Casually ask at the start of the rental (about 30 seconds into the transaction, maybe--after he's had a chance to pull your reservation up but before he's had a chance to establish control of the transaction) what kind of car you'll be getting. If he mentions a midsize or above, score.
- If it's a Caliber or other compact car, ask, "Is there any chance I could get something a little nicer if I buy a tank of prepaid gas?" (I pick gas, because it's the least costly thing to get and you can use a full tank and actually come out ahead anyway.)
- If he says no, ask how much more a bigger car will be. This way, he'll know price is an issue for you and will (at least subconsciously) keep the offers lower than he otherwise might.
- If he offers you a fullsize or SUV, say, "No, that's too much. Do you have anything cheaper?" This will cause him to drop most likely to a midsize and offer it for the best price he can get.
- Depending on the price he quotes and the tone of the transaction, you can go back and say, "Can you drop that price any more if I go ahead and buy that tank of gas?"
If you play your cards right, you might be able to score the upgrade for probably around $5 per day, though be prepared for $10-15 per day more in a worst-case scenario.
Best of luck!