Hi John.

There is a bus with 7 children inside of it

Buses have no legs and children of 2 each, so there are 2x7 child legs.

Each child has 7 rucksacks

There are 7 children times 7 rucksacks per child, so there are 7x7 rucksacks. Rucksacks have no legs, though.

In each rucksack there are 7 big cats

There are 7x7 rucksacks times 7 big cats per rucksack, for a total of 7x7x7 big cats. Each big cat has 4 legs, so there are 4x7x7x7 big cat legs.

Every big cat has 7 small cats

Assuming those small cats are in the rucksacks with the big cats, there are 7x7x7 big cats with 7 small cats per big cat, so there are 7x7x7x7 small cats, each with 4 legs. That makes 4x7x7x7x7 small cat legs.

So the total number of legs equals 2x7 child legs + 4x7x7x7 big cat legs + 4x7x7x7x7 small cat legs.

Warning: if you see a school bus like this, for your own safety do not approach it!

Cheers,
Stephen La Rocque.