Yup, the eggs are very hardy little buggers. Bleach won't even necessarily kill them. My vet says that after picking up the poo, the only way to kill these hardy eggs is to torch the spot!!

Whipworm eggs are somewhat susceptible to drying, but can remain alive in moist soil for years, and are resistant to freezing. Because of this, animals should be restricted from contaminated areas. There is no effective method for killing whipworm eggs in the soil. The only alternative is to replace the soil with new soil, gravel, and pavement.
From http://www.peteducation.com

I would suggest doing a fecal float on any and all incoming fosters. You may want to consider deworming them before arrival (regardless of fecal results) just to be safe. Dewormers are generally quite safe.

I would keep your dogs on one of the following heartworm combo meds: Interceptor (oral), Sentinel (oral) or Advantage Multi (topical). These three monthly meds kill adult whipworms.

Let me know if you have any further questions.