Había un hervidero de.........

[ROBINDESBOIS]

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?

[Elaina]

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

[Rusty]

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.

[bobjenkins]

Un hervidero = un montón

Usualmente en inglés se diría

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

[María José]

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