I have heard of several people using the diatomaceous earth (DE) with great success. But it was used on the lawn and in the home on pet bedding, not on the actual dog, although it does say you can use it that way. It does say not to inhale the dust so I would avoid that at all costs.
If it were me I would only use the DE on the yard and bedding to be on the safe side.
Here is a natural oil that you can put on them and it is suppose to be completely safe and effective.
Critter Oil Flea Repellent
Between these two things I would think they would be well protected.