A 'good' microscope is a spendy proposition. Most of the models aimed at kids are low quality and difficult to deal with - a combination that can turn kids off.
In the past five years simple usb digital microscopes have emerged. The optics aren't great, but they're sometimes easy and fun to use. Just connect them to a PC or Mac and play away. Heather bought this one for her eight year old daughter and likes it. $35 and it is a favorite toy.