If you google "chemistry kids" or "science experiments kids", you'll quickly be drowning in tons of ideas (I used to have hundreds of these bookmarked, but they have not been called for on the newest computer, alas, so i'm having trouble finding our old go to's). Our absolute favourite experiment was "hot ice", a supersaturated, supercooled liquid that will instantly solidify if you joggle or touch with a seed crystal. The science behind, from how clouds work to chemical hand warmers, kept our DS busy exploring for weeks (plus finding ways to sneak "Bose Einstein Condensate" into every conversation for months).
You can't make too much Ooblek, either.
http://static1.squarespace.com/stat...06-02ClassroomChemistry-HotIceMHNews.pdf (not the best site, but a good basic description of hot ice)
http://www.housingaforest.com/dancing-oobleck/ (best ooblek site out there!)
http://www.chem4kids.com/ (and related science sites are good starters)
http://coolsciencevideos.com/ActivitiesDemos/Activities_Demos.html (another good starter site)
