Can I apply 社會安全號 for my wife.

I am working in the US with H-1B currently. I have got a social security number long time ago. Now I am trying to move my family from Canada to US. I went to a SSN branch today to try to apply SSN for my wife/kid. But the SSN office told me H-4 (wife/kid's visa) could not apply for the SSN number. Does anybody has a experience with this situation. What should I do if I like to get the SSN for my wife/kid?

Thanks.