We (couple with 2 children) booked one hour in advance on a Friday night in late September and had no trouble reserving a table outside (small inner courtyard). The place is not too easy to find, we had to use our GPS on the phone.
We shared a couple of fish appetizers and everybody had a "secondo with contorno" (main dish) which was both excellent and reasonably sized. The two desserts were top notch, especially the hot chocolate cake. Staff was friendly and had no trouble with our requests to accommodate a few changes in the menu we required for our kids.
A few notes: there is not a very wide choice of beverages for young children (coke, fanta and orange juice) and the service was not as quick as we would have liked (kids were getting tired after a day walking here and there).
Price was not too cheap but still reasonable considering it's Venice. Overall, highly reccommended.