[DoughnutHole]

Why not speak to them in both? If they're near adulthood already it might be too late, but if they're young kids are basically sponges for languages. And growing up bilingual makes it much, much easier for them to learn a third language down the line if they ever want to.

Speaking to them in your native tongue isn't going to make them worse at English if they're growing up in an English-speaking environment.

[FredPret]

It takes a village to teach a language. At best I can teach them a smattering of it

[zarzavat]

My father's parents were immigrants and they spoke their native language to each other, but they didn't want their kids to learn it. I've tested my father and he has about a 5000 word vocabulary, pretty good for someone whose parents didn't want him to learn it!

Kids have an incredible ability to learn languages, given motivation, and they don't need to go to school to learn words like "accountant" reading books will do.