Getting the umbrella is good, because during the process they review the rest of your insurance (at least they did for me).

>stay at home mom

I'm in the same situation, it sure would be nice. We do both have life insurance at least.

>Why do you say this instead of just relying on the liability coverage under your regular auto insurance?

To be careful. If you don't buy it, make absolutely sure that your own auto insurance covers it. [also people who don't own a car won't have auto insurance..]