Il est should be translated as there are. It’s a literary usage for il y a.

Here, vieux enfants just means adults. Since he was just talking about a child, the author is following it up by metaphorically referring to the adults he’s comparing him to as aged-up children.

In essence, he’s saying there are adults who never followed up on their childhood “promises” (possibly he’s really meaning childhood dreams, but there’s not enough context to tell).

Vieux enfants means grown-up people in an oxymoronic way, as rightly stated Erwann in a comment. Du moins means “at least“, not “but”.

There is also a second play of words between qui n’ont pas fait leur devoir (who didn’t do their duty) and the more common expression when talking about children qui n’ont pas fait leurs devoirs (who didn’t do their homework.)

Note that j’aime bien is lighter than j’aime, similar to “I like” vs “I love”.

He is an attempt to translate this text:

I like this child. At least (s)he speaks his/her mind. Some other ones didn’t keep their promises, there are grown-up people who didn’t do their duty.