Translation:I show the children the small spiders.
apparently an idiomatic expression conveying that we would recognize those spiders by their smallness, and that they do not just happen to be small among other things.
But does this mean, that with "cei" we can then use the definite article -ii without having introduced those spiders in a previous sentence?
Like in English "I show him these (famous) little spiders"?
I don't know if this helps, but you could think of "păianjenii cei mici" as "the spiders that are small." So, you don't show the children the big spiders because you don't want to frighten them.