We have two children of Grade 3 and 4 and we have to find the probability of these two.

$$ Probability\ of\ grade\ 3\ child=\frac{1}{20} $$

and

$$ Probability\ of\ grade\ 4\ child=\frac{1}{15} $$

$$ \therefore Probability\ of\ both\ children=\frac{1}{15}\times\frac{1}{20}=\frac{1}{300} $$