There is no such thing currently as reducing the risk of killing yourself or someone else with your car to 0. Braking distance and speed are insufficient variables to alter given people's preferences about the speed at which they'd wish to arrive at their destination. This will very likely also be the case when autonomous vehicles are deployed. There will be collisions, this is simply unfortunately a fact.

Given that there will be collisions, it seems prudent to have the car try to figure out what's better to collide with- the tree or the child. Given that there will be collisions, it also seems prudent to think carefully about other, more difficult moral dilemmas.

Refusing to code for an eventuality is itself a moral decision; inaction is an action that has an impact on the world.