Doing a little more research, those rice grain-like segments that are shed out of an infected animal's butt DO contain tapeworm eggs, but the good news is that a flea has to eat the tapeworm egg in order for it to develop into something that can infect another pet. The flea, an intermediate host, is required for the tapeworm to complete its life cycle.
Any vet can prescribe the appropriate dewormer, but if you want to wait until you get home, Sophie should be okay. After Sophie is dewormed, clean her sleeping areas carefully so another flea doesn't gobble up an egg and give Sophie another tapeworm.
And don't feel like a bad mother--all it takes is Sophie eating one flea, and it could have happened anywhere. It doesn't mean that Sophie has a flea infestation.
Here is another article about the tapeworm life cycle:
Tapeworms In Your Dog Or Cat