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.
> 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.