No way I would (or you should) let the children lie in the car without seat belt or even butput them between luggage, which could move and sqash them in case of an accident.
And - as torgengb correctly remarks, there are many alternatives to a frontal collision with very different effect on deformation of the car and movements of luggage and people inside!
And even if the luggage is in front of them as long as you drive normally, you can never know what happens in case of a crash, as you can not guarantee that everything is fixed so tightly at its position that it can impossibly move during an accident!
-> The solution with the least risk for injury are the child seats!
what you can do is
- use pillows to support the neck
- incline the child seat in a sleeping position, if it has one
- try to split up the long journey by making a stop somewhere
- be careful to avoid accidents (although you use the child seats)
If your child seats are so uncomfortable, maybe you can borrow a more comfortable seat just for the duration of the trip? At our national automobile club (in Germany) it is possible to borrow child seats for some days or weeks. (However, I fear that it might not be much cheaper than buying one, if its not only for one or two days.)