Zero hours contracts in theory mean you can refuse work (at least that’s how it works in the U.K.), but obviously you’re going to be fired for doing so, so you’re essentially constantly on call for both/all your jobs.
You're saying they should keep the Walmart job and get rid of the other, and spend the time freed up searching for one that's full time and paid enough to replace both, right? But if you can't support yourself and your dependants on the Walmart job alone, then that's risky.
Look, here’s how it goes:
1) you have no job
2) you take the first job that comes your way, even if it doesn’t actually pay enough to survive - a little money is better than no money
3) you take the second job that comes your way so that you can survive
4) you have no time to actually do anything towards getting a better job
Understand?
This is why jobs that pay less than what is needed to survive are bad for individuals and bad for society; people have to take them. Nobody is going to help you survive otherwise.
Searching for a job that is _mechanically better_ (think office job versus min-wage retail) is significantly more challenging than finding a min-wage job. For example, most salaried or otherwise higher paying jobs require multiple rounds of interviews while many min-wage jobs will hire you on the spot to fill a staff position.