Try these tips: https://www.craftylittlegnome.com/kill-fleas-in-the-yard/
These are our scientifically proven ways to get rid of fleas in your yard. These tips are natural and safe too: https://chemistrycachet.com/proven-ways-to-get-rid-of-fleas-in-your-yard/
You can try diatomaceous earth. The food grade kind is safe for pets.
Walmart sells a spray!
There is a brand called Adams that has a flea and tick yard spray--very reasonably priced