Preferably on a Fujifilm Crystal Archive or Kodak Endura Paper. These are protographic prints, not cheap inkjet or dye sublimation prints. If the minilab is correctly set up and uses good quality chemicals (preferably from Fujifulm, if printing on a Fuji paper, or from Kodak, if printing on Kodak paper), the prints should last a lifetime. Fuji's Crystal Archive is rated for 60 to 70 years.
I used to have my prints done at Black's (Canada), on some Kodak matte photo paper; not Endura, for I didn't have the money, but a reasonable quality paper on a reasonable quality minilab. None of them have shown any signs of fading to the present date.
If you really need the pictures to last 100+ years, take them on Ilford B&W film; if you get a good film camera, it's not harder than taking a digital picture (I have a Pentax MZ-50 SLR , which can work fully manual or all-auto, and I rarely set it to manual). Buy a film scanner (Nikon Coolscan) and scan the film. If you need prints, have them done from the film. Store the film in a cool, dry place, away from any light sources.