If you want to get the older kids into tech or they want to get into tech, you might consider the MSP430 from Texas instruments. You can get almost seven of the MSP430 development kits for the price of just one Arduino. That is right you can get an MSP430 development board for just $4.30 shipped to you in the U.S. You may want to play with it your self also. Ton's of online documentation. ( i.e. http://www.msp430launchpad.com/) Free development software is available for the most popular platforms also online (from T.I.).
What is really interesting about this unit is that you can take some software source code for the Arduino and run it on the MSP430. Porting software to the Arduino should be easier if you plan to get the Arduino later. The unit even comes with extra dip chips for your use.The newer Arduino units are soldered in place so really can not do complete development on the unit alone. Even if the kids grow tired of it, you have not invested a fortune in the unit. Win-Win situation.
At the holidays, they may sell out again, so order early.