An easy hack, of course, is to tie the elastic strings, but sometimes that makes it too small! Plus this silicone ear-saver is anti-slip to keep it locked in place, and designed not to get tangled in hair.
Psst — a lot of reviewers mention this worked great for kids' masks, especially since sizing for kids' faces is all over the place and difficult to measure out when you're buying masks online!
Promising review: "Yay! These work as well as I hoped they would for ear-loop style masks (with elastic loops)! I have smallish ears that are a little more diagonal than usual, so some masks slip off my ears too easily, and others droop and gap a bit. I was looking at everything from velcro cord organizers to bra extenders and crocheted ear savers with buttons (those are pretty good too), for a better way to secure my masks. I also wanted to find something suitable for my older relatives with dexterity issues. These are the overall winner! They feel sturdy, and the multiple knobs make them very adjustable. I've used them with KN95s, pleated procedure style medical masks, and handmade fabric masks that have elastic ear loops." —CJ
Get a four-pack from Amazon for $6.99 (available in eight different color combos).