I think the dialog is better here, though at times they still don't quite sound like children. I liked that you built up the scenery and the environment better, and have described a few more characters.

You're a bit inconsistent about referring to characters by first name, first and middle name, or full name. I know Italian names are a little different, but you should generally stick to one form to refer to a character (once his or her full name is given), unless there's a particular reason (such as emphasis) to provide a different form.

Lapo and Maria Cassia were both ten, and like Ricky, they were too young for Beauxbatons or Durmstrang, but far too old to be left in empty houses alone.

This implies that if they were younger, they could be left in empty houses alone. Which I don't think is what you meant. (Ricky notwithstanding, and apparently he wasn't really left in an empty house -- he had a house-elf. Which reminds me: why didn't the house-elf do anything while Ricky was dragging chickens into the living room?)

Flat and slightly greasy mousy hair falling lankily over a pale, pasty face from which grey-brown eyes peered out with a constant expression of sarcasm and superiority, a weak chin, and sickly red lips that looked as though all the blood had drained there from the rest of the face; an affected, intolerable drawl; and a constant habit of calling the younger boy “kid” or “runt” or “shorty” or the like, and generally treating him with barely veiled contempt.

This is a great description -- but grammatically, it's a sentence fragment, and a very long one. Also, too many adverbs. And the word you want is "lankly," not "lankily."

“Why should I, runt? Because truth hurts?”

So, here's a really nitpicky point: this is an example of where the dialog is just a bit... off. It's probably due to the way it sounds to American ears. I don't know how this would usually be expressed in British English, but we'd say "...the truth hurts." "Truth hurts" is grammatically correct also, but using "truth" as an abstract unquantifiable noun in that sense, in this expression, just doesn't sound quite right, and makes this ten-year old sound a tiny bit stilted. It's little things like this all through your dialog that sometimes throw me off; it's rarely anything that's ungrammatical or hard to understand, it's just not the way casual conversation sounds.

So, Ricky and Lapo are going to get themselves in trouble... I continue to enjoy the fairy tale nature of this story.