Do they not have Social Security Numbers at all?  Or can your client just not find the SSNs?

Do they have ITINs (which are usually issued for people who don't have and don't qualify for a Social Security Number)?

If the spouse and kids don't have SSNs or ITINs, it would likely be beneficial for them to apply to get ITINs so they can be included on the tax return.  I would refer them to a Certifying Acceptance Agent to get the ITINs.

https://www.irs.gov/tin/itin/itin-acceptance-agents