The question is on the highlighted line of the Moralité to Le petit chaperon rouge as collected by Perrault.
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.
Je dis le loup, car tous les loups
Ne sont pas de la même sorte :
Il en est d’une humeur accorte,
Sans bruit, sans fiel et sans courroux,
Qui, privés, complaisants et doux,
Suivent les jeunes demoiselles
Jusque dans les maisons, jusque dans les ruelles.
Mais hélas ! qui ne sait que ces loups doucereux
De tous les loups sont les plus dangereux.
What exactly does it mean?
How does it come to mean what it does? (See Background below.)
If today's French would say it differently, how would it go?
For 2, I have come up with the following wild guesses, which may illustrate the kind of sense I would like to make of the line.
A. en as pronoun
By this guess, en stands for the young children and young girls, and the whole line becomes:
S’il est tant que le loup mange de enfants et de filles.
This presupposes many things:
manger de would have to be available as an expression (maybe somewhat like the English expressions eating of this bread or drinking of this blood).
en would have to travel outside the que context and end up between il and est.
tant que has to serve as a conjunction. Maybe it means what it seems to mean and we get:
If it is so much (the same) as the wolf eats of the children and the girls.
B. en not as pronoun
By this guess, en does not refer to anything, but is simply part of a set expression il en est tant que or perhaps even s'il en est tant que. If so, it becomes a matter of learning what this set expression means. I am not able to find the exact thing, but may be the following are related.
si tant est que means supposing that or even if according to this dictionary entry;
en tant que means a number of things according to this Wiktionary entry.
From those expressions, I may try to guess at the meaning of the line in question. For example, pretending that s'il en est tan que means the same thing as si tant est que, we get (for the whole line):
Even if the wolf eats.
But eats what? It is unsatisfying to see it end like that. In fact, any solution that includes en in a set expression would let the line end in the same unsatisfactory way.