how do you keep your common law wife who has a child with you from getting SSI from you if you die?

Why would you care after you're dead? And you probably mean social security, not SSI. If she is truly your common law wife, you don't keep her from getting it. Living together and having a child together doesn't make someone common law married though.

did you write this question backwards?

Get divorced and marry someone else.

Marry someone else.