The amazing wonders of built-in meta-grammatical awareness

I try to read books daily to my kids, and in a perfect world,  I should read only in Italian to them. The reality is that, when I come home after a long day at work, tired and famished, and they ask me to read one of the many French books they love from their library, I just read it in French: my brain, on certain evenings, refuses any collaboration, and if I try to simultanously translate in Italian, I sound like a foreigner! The kids don't mind it, but while Zeno keeps interrupting me with 1000 questions about the story (that is, focusing on the content of the story), Milo cannott help correcting the occasional pronunciation mistake I make (a nasal vowel, a missed liaison...).
Tonight , though, he went a step further and he simply blew me away! I was trying to read the following sentence:
"Ils entrent dans la salle..." which means they enter the room; while the final 's' is normally not pronounced in French words, in this case it needs to be pronounced because it provides a liaison which helps the listener capting the plural nature of the verb.

So I stumbled a few times around the sentence: "Il rentr...ils rentr...il rentre?" and Milo shed the light for me:
"Si dice Ils rentr, mamma: se fosse stato uno solo avresti dovuto dire il rentr, ma siccome sono due..." (You read it ils rentr, mama; if it was just one person you would have read it il rentr, but since it's two persons here...).

The kid is still in kindergarden (he will turn 6 soon, and start primary school in September), and while they work a lot at school, grammar is definitely not on the program yet!
How could he come up with such a logical and grammatically oriented explanation for something he knows only by ear, in theory?
I'm stille in awe!