My wife and I are french and we live in the US.
Our 3 1/2 boy goes to day care in english. His english has become quite good, almost on par with his american friends.
At home we speak french (with some occasional slip up). He understands us perfectly but his spoken french is very limited. The situation now is that he speaks in english or a mix of french-english to us and we answer in french.
We have not followed any method (as you can see
Shall we refuse to answer when he speaks english to us ?
He does seem to get frustrated when he can't express himself in french.