A car is an asset. Just because it has maintenance costs does not mean it is a liability. You can sell the car, keep the money, and have no more costs to pay. If the car had a loan attached that you were expected to pay, then it would be a liability.
The kids could use the car to go to their job(s), or keep you from having to drive them, so you can spend more time at your job, and make up for the difference.
If the kids do not NEED a car, then I can see where it might be a burden. In that case, why not ask for a cheaper car and, say, a year's worth of gas money?
Since we have just started to save as much as we can, the idea of getting a new car goes against our current ideology.
What are you saving for? What if you ask for a contribution to that goal, and when you reach it, then you can start saving for a car?
Either way, if the kids already know that they are getting a car, it might be a VERY difficult conversation if you decide that you want something else. In that case I would probably just accept the generous gift and figure out a way to make up the expenses that go with it.
Finally, don't overlook the interpersonal aspect of this. Your in-laws want to do something incredibly generous four your children (their grandchildren). I would be very careful not to treat this gift as a punishment just because it was not in your financial plan. If this car will create a strain in your finances then you can have a conversation with them, but be sure that you acknowledge the significance of this gift. If you don't show the proper appreciation for the gift, you may find yourself with nothing but an awkward relationship.