Sweet little Joey! Your son deserves accolades for having the presence of mind to go check the adult kangaroo after an accident. Glad to hear your son is okay and that the joey survived.
In the States, deer ARE a problem. We have two plastic gadgets that fit on the front grill of the vehicle. The air goes through them creating a sound not heard by humans, but it turns deer right around. They are called, "Whistlers." Wonder if there's anything that would help with kangaroos?