I don't even think you need to treat the lawn. We were an indoor only cat family for 9 years (before we adopted an indoor/outdoor) and never needed to treat the lawn or our cat. I simply combed our cat and kept my eye out for fleas.
In the 9 years, she only had one hijacked flea (that we must have brought in) which I caught early and never had a chance to lay eggs or infest the whole house. At the time we lived in a duplex and our downstairs neighbor had a dog and cat with fleas. All the subsequent years, we lived in a larger place with grass everywhere. . .never had fleas. . . .never treated the lawn.
Now that we have an indoor/outdoor cat, we treat both of them, spring through fall with Advantage. If you do get fleas, don't worry about the Advantage. I read a vet who said that the only bad thing that will happen to your cat if it licks off some Advantage is that there is less stuff to kill the fleas
If your kitties do get fleas, you'd probably only need to treat them one time anyway.
Treating a whole lawn will also probably kill all your friendly bugs and earthworms (my favorites) and may not be necessary. Why not give it a try this year and see how it goes? Just an idea.