Assuming the die is fair, on the first roll, the child has a 1/616 chance of getting any number, including 6. On the next roll, the child has a 1/36136 chance of getting any two numbers, including two 6's. This is because the rolls are independent, so the total possible outcomes multiply each roll with each roll's probability. Since each roll's probability is 1/616,
1/6 * 1/6 = 1/3616⋅16=136
Finally, the same thing happens again on the third roll, the probabilities multiply once more, so
1/36 * 1/6=1/216136⋅16=1216, meaning there is a 1/2161216 chance of getting any possible combination of three numbers from 1-6. This includes getting three 6's, therefore the probability of getting three 6's is 1/2161216
I hope that helped!
