> The solution (if they're still babies) is to simply speak to your kids in your first language all the time.
No need for them to be babies. That will work as long as they're younger than 12, and probably for a few years after. But if the kids have already learned to speak another language, they will hate this approach.
> But if the kids have already learned to speak another language, they will hate this approach.
The way to fix this is to find them things that only exist in that language and not in English. It could be other family members, friends, TV, music, time abroad, etc.
Kids, like adults, need motivation as the primary factor that determines success learning a language.
No need for them to be babies. That will work as long as they're younger than 12, and probably for a few years after. But if the kids have already learned to speak another language, they will hate this approach.