We got DS6 a microscope for his 5th birthday. Its the My First Lab DuoScope. Aculady is right- you can get some that will last up through high school science for a couple of hundred bucks. If, like us, you are looking for one that is pretty fully featured but not as powerful and doesn't quite have the highest level optics but comes at a good price there are a few to be found. (We didn't trust that our wonderfully bright but often clumsy 5 yr old wouldn't accidentally wreck a $200 piece of equipment)

Check out this link:
http://microscopes.toptenreviews.com/kids-microscope-review/