Leanne,
that sounds like great fun, trying to balance the initial push to give enough speed that friction will slow them down to stop at a given point.
Nice possible links to stopping distances for cars in the dry vs the wet, and lots of opportunity to look at how well the wheels turn on the vehicles.
I’d take care that the children realise that once they have pushed their car, it will keep moving unless another force (in this case friction) acts on it. I’d consider trying to make a really slippy table (furniture polish) and let them explore the frictionless world. It is all to easy for them to conclude that they have to ‘keep pushing’ to keep the car going. In fact, they have to keep pushing to overcome friction.