Fleas and worms

I don't think fleas can live in grass easily, if at all. They like living in fur I think, more than grass. If you have a dog they might put fleas on the lawn which might get on the guinea pigs. You could put a tiny bit of that pet safe insect repellent (not insect killer) onto their coats (spray it on your hands then rub them with it).
As for worms, I really don't know.

I would certainly steer clear of insect repellent. Even the natural ones are citronella based and I'm sure would irritate the pigs.

If you are worried about worms, oral ivermectin can easily be used to treat them.

You can also treat flees on guinea pigs with Advantage. Advantage will also treat lice and ticks. It is good for one month providing you don’t bath your guinea pigs. You apply it behind the ear. Depending upon the weight of your pig, will depend on how much your pig should get behind the ear. Also, Advantage is the same concentration for all animals, just depending upon your pets weight will determine the correct amount to give. You can also treat your cat and dog with Advantage. It is applied in a different spot for your cat and dog, then your cavy.

I thought flees really don’t like cavies? Was I under the wrong impression?

There is always a risk of your piggy catching worms as various things can carry them, including birds and mice. As someone said earlier it is easily treated with either oral medication or ask your vet about other treatments which include a small drop of treatment on the back of the neck, though an injection is sometimes used as a more reliable way but should be used as a last resort.