My son started enjoying Minisebran at about that age. At first he would enjoyed mashing the buttons and seeing different shapes come up. At the age of about 2 he started being able to press the key corresponding to the letter on the screen. And later still as he got the hang of the mouse he enjoyed colouring and drawing (at which point we introduced him to TuxPaint and Poisson Rouge). We've never bought any special input devices for him though.

There is a nice applet at http://www.falstad.com/circuit/ - it might not be sufficiently sophisticated but it does at least handle op-amps.