Our local library has been a great resource for us - science fairs are so big in schools that the library has a large collection of kids' science experiment books all grouped together in one place, and we've used those as a jumping off point for one of my science-kids. She likes to look through the books for ideas. My other science kid (my EG ds).. came up with his own experiments and ideas as a young kid and didn't care about getting ideas from anyone else

FWIW, I'm a scientist and so is my dh. Most of the early elementary level science experiments my dd has looked through have come with good explanations of the concepts behind the experiment and the simple experiments are to a certain extent easily reproducible so you don't have to overly-worry about them not working. If you're moving on into more complex ideas I'll second HK's advice - it helps to have someone working with your child (parent or other person) who understands the science concepts being tested. OTOH, I think that simply being enough of a scientist to realize not every experiment "works" (and that most actually "fail" - ie don't prove the original hypothesis), but every experiment, no matter how it works out, is giving you data that means *something* - that plus a little bit of patience, is really what's important. If you run into something you don't understand, you can always seek out someone to help you figure it out at that point.
Have fun!
polarbear