Looking at their situation, $20/day for 4, probably means husband & wife need to work to make ends meet. Hence, home cooked meals may not necessarily bring down the costs.When you only have $20 to spare a day for your family of four, then you must start to cook, rather than eat out.
Unless you say, don't know how to cook and must eat out, then I can tell you, you tell the economical rice stall to add more rice and order veges.
There's gas (cooking) & electricity (refrigeration to keep ingredients fresh, and preserve the packed meals) costs involved. The children with lunchboxes may not have means to preheat their lunchboxes in school.