So first of all, you can’t really separate the sentence. Here it should be read in French as “On voit ici que de jeunes enfants, surtout de jeunes filles belles, bien faites, et gentilles, font très mal d’écouter toute sorte de gens, et que ce n’est pas chose étrange, s’il en est tant que le loup mange”. The last part, the one that you asked about means it’s not surprising that the wolf eats so many of those girls, since they listen to all kind of people.

In modern French the sentence would be usable, yet not commonly, it would be “language soutenu”, which means educated people, high class and scientists would use it. In more common French “Et ce n’est pas si étonnant, que le loup en mange autant.”.

Hope it helps

1 – 2 / "Il en est" is a rather archaic equivalent of "il y en a" : "there is / are". "Tant", in this context, means the same as "autant" : "so many / much". "Tant que" also means"while" but that doesn’t work here. Putting that together word by word, this phrase means "if there are so many that the wolf eats". If you take what comes before, the meaning becomes clear :

We see that young children, especially girls […] have a bad habit of listening to all kinds of people and it is no strange thing if the wolf eats so many of them

More simply : young girls listen to strangers, no wonder they get into trouble.

3 / In modern French, the phrase as it is could be found in literature or poetry. It is quite formal and literary (and even a bit pedantic) but correct. More common ways of phrasing it include :

S’il y en a autant qui sont mangées par le loup

Si le loup en mange autant

"S’il y en a autant que le loup mange" would work too, but it feels a bit awkward. It’s probably a matter of personal taste and style, though, there is nothing really wrong with it.