Hi, Kristin. Sometimes things like this can get pretty involved because you don't really know what you are dealing with until you get the plates off and sometimes until you start getting the paint off. Here is a method that should be pretty safe.
Get a cheap aluminum foil pan that is big enough to hold all the plates flat. Remove the screws that hold the plates on the door by digging out the paint that is in the screw slots and then use a screwdriver carefully so you don't scratch the plates. Carefully cut the paint that is probably holding them to the door with a utility knife but try to not cut into the door wood. Pry the plates off with a putty knife.
Put the plates in the pan and pour some paint stripper over them. I prefer a safe stripper that is non-toxic, just in case. My favorite brand smells like oranges. Wait until the paint gets all wrinkly. This will probably take a couple of hours and you can leave the pan outside but out of direct sun.
Remove the plates and use a stripping pad to get most of the paint off. You probably will not be able to get it all but you should get most of it. If there seems to be a lot, dunk the plates in the stripper again and then the stripping pad. If there is just a little, use a brass brush to get the rest of the paint. You can leave the plates in the stripper overnight or for 24 hours with no problem at all, if you wish. When you are through with the stripping, wash the plates off well with paint thinner.
Depending on the kind of finish you find on the plate, you may want to polish them. You can use jewelers rouge on a cloth wheel in your electric drill to do this easily. Don't go too hard at this as you might go through a plating layer. If you want to keep the shine, spray the plates with a couple of coats of spray lacquer. Again, there are so many different styles and materials for plates that I can't give you one answer for how to treat them. Some plates had a black oxide finish, for example.
Good luck and let us know how this comes out. Henry in MI