The early termination fee is just that - a fee that they assess you for ending your contractual obligation with them earlier than expected. It's virtually impossible for you to talk your way out of, as the specific legal language surrounding the fee is already agreed to by you (or rather, by your parents) when they initially accepted the contract with AT&T.
In other words, you aren't going to get out of paying the fee if you want to leave early, unless AT&T does something to invalidate their contract with you (which is about as likely as you winning the lottery and being struck by lightning in the same day).
Now, there are ways to get around it - for example, buying a Sidekick directly from an online store or eBay that is unlocked. Such a phone would work with AT&T, but the internet wouldn't work. Or, you can find a phone that is already programmed with AT&T as the default network - this will basically mean you just change the IMEI number AT&T assigns to your account (you call them, tell them you want to change your number to a new phone - easy process) - and the phone will be completely and fully functional.
Odds are good that if you have to have a Sidekick this is the best - and cheapest - option you have.