Couldn't agree more!
Personally, i believe that when we are kids our internal grammar (or our hability to learn a language) it's in it full develop work since we are still gathering words and constructing our native-language vocabulary. In this sense, having contact with another languages as kids, would make more easy to notice the aspects of that other language and understanding it's parameters/structures as well as ours. Definetly, the sooner the better!