If you have multiple players but only a single smartphone, you can only login to one account at a time. You can plan ahead by downloading alternate Web browsers for your smartphone operating system (feel free to post suggested apps!) to be able to handle more than one by just switching browsers.
this works great, thanks for the suggestion!
i just registered my son on pokygym and added his player ID. then use the default browser for his ID and firefox for my ID to access the stadium page on my samsung droid charge (dolphin's been crashing on me lately), and just switch between the two browsers. nice!