I have a 4S and have been using it, but it's very frustrating. I am unsure of how much of it is to do with the app/server issues and how much from the phone.
In order to use it I have to make sure no other apps are open/running in the back ground, and that the app has no access to the camera, if you attempt to use the camera it's over. And forget using a gym - this always causes it to crash.
Sometimes it has trouble loading, but it can be bypassed by putting the phone onto aeroplane mode, and then turning it off when the warning for no connectivity comes up. It sounds like a pain, and it is, but it doesn't seem to bother my 7 year old too much and he does it himself. If I turn the screen brightness down, the battery lasts around an hour before going flat.
So, is it a desirable device for pokemon go? no. Is it adequate for a child? I would say yes, but I guess it depends on how well they deal with frustration when the app crashes in the middle of catching a much wanted pokemon!