That wasn't an error, but the author made a few small errors (really, just unidiomatic phrases), which suggests he or she is not a native speaker of English:

> it was such a pure horror

> a happy end

> it did not wave the arms

> made no swimming movements with the legs

> a dozen of pools and slides

> 7 boys in that age

> not loose them

None of these are a big deal, though, and the writing was perfectly understandable.

Sorry, I am not a native speaker. German. Will fix the comment, so everyone can ease off again...

You're doing well, it was just someone being a grammar stickler for no good reason.

Or someone correcting him so he can learn better.

Sorry, I didn't mean to start a grammar war. It was just something that stood out as a bit strange, the meaning was understood fine though.

Clearly not a native English speaker. They exist.