"You might wait to see the portion. Portions are generally much smaller than the typical Am portion."
Exactly. Only once, in Siracusa in Sicily, have I seen large American portions in a restaurant in Italy. Judging by the reactions of the other diners when their dishes were brought out, the locals were as surprised as I was. Everywhere else, the portions were much smaller than in the US. I can get a primo, a secondo, and a dessert - and still not be too full.
To answer your question directly, no, adults can't order children's portions. But you can, as everyone said above, order fewer courses, since everything is a la carte.