Your child may think that with his calculator, his answer would be invincible. But I have seen children keying wrongly on the calculator before.
So, even when using a calculator, it is good practice to punch in the working from scratch again. The chances of hitting a wrong number twice is rather low.
This is such a good practice that banks mandate their tellers to do this too.
Experiment #7 - Forces (Frictional Force)
Experiment #6 - Forces (Frictional Force)