I feel like I've had this talk, but in small bites over the years. Usually, the question of "why do you have to go to work every day?" gets the simple answer "Well, I need to make money." which usually just creates more questions anyway. Sometimes the question is "Why do I need to go to daycare?" and the answer is "Well, because me and your mom have to go to work." Sometimes that generates another question, sometimes it doesn't. When it does, like "Why!" the answer is the same: "Because I need more money for you kids." At some point, they'll ask about why we go to the store every other weekend, which has a simple answer: "Well, we need food to eat, don't we?"

You get enough of these questions over time, and you end up talking in a loop. "Why do we have to go to daycare??" -> "Remember how we went to the store this weekend to get all that food?" -> "Yeah..." -> "Well, I bought that food with money, and you have to go to daycare, so I can go to work, so I can get more money".

You hit it with them all at once, and they're going to just glaze over. Or you'll be in the middle of it and then they'll tell you a story about how this one time when they were at daycare they saw their friend and that friend had a dinosour t-shirt on, and they like diosours, so they stomped around pretending to be dionsours, and that was a lot of fun, and oh what are we doing today and blah blah blah blah blah, what were we talking about?

I think at some point I was even asked why I have to work all day, and my answer was something like "Well I give up some of my time each day, and my job gives me money for that time I give up." something like that. At some point you'll realize you've explained most of the basics of the system to them.