Give 'em a shot. I mean, My Neigrabroador Totoro, for example - Dakota Fanning and her little sister? Wouldn't have been my first choice. But they did a fantastic job - as good, if not better, than a lot of professional voice actors. And even if the main cast is mediocre, the supporting cast looks pretty sweet. They got Liam Neeson. And Cloris Leachman, too; she was one of the best in Laputa, no doubt.
Plus, even if EVERYONE sucks - it's a Miyazaki movie. You could watch it mute and it would still be a beauty.