You can put it out on social media which costs nothing if you don’t count your wifi cost as part of it. Flyers around the neighborhood would be great too; you can bike and tape the handwritten signs to stop sign poles for example or near garage sale signs. Billboards seem a bit too excessive for a lemonade stand. You should have your stand at a place where people are going to be outside most of the time, like a shopping center. If you pledge to donate 5% of your sales to a charity, then that can also bring in more customers. In terms of advertising though, word of mouth helps too, as well as the exposure listed above.
Hope this helps.