I know that my eyesight has worsened by being in front of the computer 8 hours a day (at least). I'm a transcriptionist and have to stare at the screen for hours at a time. I'm not familiar with SesameStreet.com. I have teens. Maybe you can limit the number of games she plays. If what she is doing is a task to completion, say one letter game, then a number game, etc., then I would find a definite start and finish, and make that her limit. If you make it a time limit, for example, she may still be in the middle of a game when the time is up, in which case, she has a bit of leverage on you. She'll want to finish the game, and really, that would make sense. If you give her a finite number, one or two games at a time, then help her with some sort of method to help her count down how many games she's played, maybe using a chart, or by dropping a ball in a fun container when she completes a game, it will help you have very clear start/finish guidelines that a 2 yo can understand. Even though this is a game, over time, she will actually learn very basic organization skills: setting goals, completing tasks, keeping track of what's been completed.
So, use it as a teaching tool, limit it to protect her eyes and to help her from developing a gaming addiction. :)