You can teach budgeting to your student by making them a part of your budgeting process. Show them on paper (you may want to use round numbers for easy calculations) what money comes into the household, what money needs to be spent on household needs, how much is spent on wants, and how much is saved.
Then when you’re out running errands with them, put your student in charge of telling you how much you can spend at each store and ask them why that is the amount you’re able to spend there. By asking them why, you push them to think critically about how that number was generated and what that number means in a real-world context.
You can also put them in charge of keeping track of how much you spend at the store, to build your student’s money-management skills. To have your student continue to think critically, consider choosing items you don’t need so your student has an opportunity to tell you to put them back. Ask your student why you must put that item back, and this will give them an opportunity to apply their lessons on wants versus needs, value, and staying within a budget.