Había un hervidero de.........

Había un hervidero de.........
 

Un hervidero de niños = había muchos niños
Un hervidero de serpientes = había un sin fin de culebras.

WHat is the equivalent of Hervidero in English?

I've never heard of "hervidero de niños"

I've heard of the word "hervidero" but used only for bugs.....

pile of
nest of
bunch of

English has many collective nouns (what we call groups of animals, insects, birds, etc.). Have a look at some of them. If we don't know the collective noun, we can always say a bunch, tons, a slew, a lot, etc.

We quite frequently say a bunch of kids, though we can get inventive and use other words, too. ;)

My dictionaries say hervidero is a swarm of insects, or a seething mass of people.

Un hervidero = un montón:confused:

Usualmente en inglés se diría

A bunch of kids in the park..
A ton of kids in the park

The English word swarm is very similar to hervidero
P.S. Just noticed Rusty had already said it...