Give a look at: http://www.hobbytron.com/electronickits.html
Actually go to GOOGLE, and type in ELECTRONIC KITS
or better yet type KIDS ELECTRONIC KITS
you'll find hundreds of suppliers that offer everything from VERY VERY beginning kits that blink or make noises... to some quite advanced kits that will challange your child and perhaps even your "inner child".
Happy Kit Building.