I've heard mushrooms can be no good for dogs. So if there was some mushroomy-type growth on that wood, it might be that. And also there is the possibility they either have got blockage caused by pieces of rotten wood or like Margot said -splinters.
I think I would take them to be checked out -be on the safe side.
Did you by any chance see any fungus growing on that old tree stump? Or, another thought is -what kind of tree was it? Then you can find out which growths are more likely on which species of tree....etc. That might give you a clue to what they ate.
Feverish? Did you actually take her temp? I would think it depends on the type of tree they were chewing on. Some can be toxic. Mold and mildew can make them sick. Some of these things (like green or rancid walnuts) can make them very sick and cause seizures. I would think, if they have not seized by now, they probably won't. When in question, you can always contact the Animal Poison Control (that may be listed in the front of your phone book) and run it by them. So, hard to say not knowing what they injested. I would keep a close eye one them or possibly give your Vet or ER clinic a call and see what they say.
Had a friend who had a whole litter of pups seizing and she didn't know why. Finally figuered out green walnuts had been falling into their drinking water outside.
Hope the dogs are going to be okay but I'd do a bit of checking to find out if they need to be checked or given something in the way of meds possibly. It's too late to induce vomiting.
If they were chewing on wood, I would be a bit concerned about wood splinters. Keep a close eye and make sure they are passing stool and there is no blood in it.