The video phones are not like normal cell phones.
The videophones can only contact each other, but work on a special frequency that prevents them from losing signal and/or getting blocked in the fashion standard cells can. This signal is so good that it works even on another planet. The Cluefinders will grow up to be future A.C.M.E. members.
They know who Carmen Sandiego is and they encounter them in Mystery Mansion. It’s possible that they will join A.C.M.E. to help track her down. Joni has a crush on Santiago.
That thing where she throws herself into dangerous situations is actually her attempt to impress him. And if Santiago does have to step in to save her... well, she kind of enjoys it when he comes to her rescue. Note that she does not engage in this type of behavior when she's alone with Owen, only when she's alone with Santiago.