Hi!
The mites didn't come from the worms but must have been hiding in the substrate that the worms were transported in. While the mites are not harmful to your toads, I would be more worried about any parasites that the worms could be carrying, so I would recommend not feeding the worms to your toads. I just did some research on mites, because I have some sort of mites in with my isopods, and I was informed by a faculty member of Penn State's Dept. of Entomology that mites will not harm toads. They primarily feed on bacteria and other microarthropods such as Springtails. My mites were not able to be specifically identified from the picture I had sent, other than they are predatory mites, mesostigmatas ?? species. So, I would just dispose of those mites, if you haven't already, and the worms too just to be safe!