I'm not a vet, Emily, so I can't answer your question. We use ivermectin as a wormer, but our dogs are much bigger and the dosing range may differ for deworming. It also may depend on whether your dogs have generalized or localized cases of mange.
But I'm pretty sure if you talk to your vet and express your concerns about the size of the dose, he'll talk to you about it. If you still don't agree with his recommendations, you can always check with another vet.
I wouldn't put treatment off too much longer, though. If demodex mange gets out of hand, it can be very difficult to treat.
Good luck with your dogs, Emily! I hope they recover quickly!