Правильное Dojo 1.7+ должно быть;
var tabs = registry.byId('someTabs');
tabs.watch("selectedChildWidget", function(name, oval, nval){
console.log("selected child changed from ", oval, " to ", nval);
});
другим способом может быть
var tabs = registry.byId('someTabs');
aspect.after(tabs, "selectChild", function (event) {
console.log("You selected ", tabs.selectedChildWidget.id);
});