How far are the cars  from the wood? How fast were the cars going? There are too much information missing..
stopping distance of small toy car is 15 in 20 cm, 18 in 40cm, 15 in 60cm, 12 in 80cm and 18 in 100cm
stopping distance of big toy car is 24 in 20cm, 38 in 40cm, 26 in 60cm, 39 in 80cm and 33 in 100cm