rio de janeiro with a three and a half year old.
Brazil is very different from the US in that way, you can safely assume that all places are kid friendly, unless your children can't seat quiet during the course of a meal or cry all the time :). Most restaurants don't have a kids menu but Brazilian menus have plenty of options that children would eat. The exception would be the very high end places, but otherwise you can basically take your children in most restaurants. The kilo style restaurants (you pay by the weight) are a very good option and one great Kilo is Frontera. Frontera has many locations at Zona Sul.
annmorris, Toot is right. Except for high end places, most places will be ok to serve kids (as long as they are well-behaved). As opposed to many places that I'v been in US, most places will be ok to substitute an ingredient of the dish, change sauces and so forth.
I think Rascal (shopping leblon and shopping rio-sul) have lots of space and they used to have coloring pencil and paper on the table... I did not like their pizza, and thought the a la carte menu was not worthy. The all-you-can-eat buffet was pretty good, but also quite expensive.