I really haven't heard enough of the kids to judge, but they sounded OK, based on what little I have heard. Sometimes hiring non-traditional voice actors can work in a show's favor, provided the actors are talented and possess the right sound.
I haven't seen any animated clips featuring the parents yet, so I have no idea what they sound like, only the voices that I've assigned them in my head. Since only Gumball, Darwin and Anais will be voiced by American actors, it'll be interesting hear how the other characters sound in relation to them.