Anyone raising bilingual child?

We speak mother tongue (MT) at home and kids (5, 3.5) are quite fluent
in MT and English. However when they speak to themselves they tend to
switch to English. I'm having to remind them constantly. They
understand the im****tance of MT because extended family is not fluent
in English and they need communicate. My siblings' kids have already
given up. They understand MT but can't speak. I don't want to get to
that stage.

Just wondering how other families are dealing with this issue.