You don't mention what grade your daughter is in but keep in mind that a straight-forward project, one which isn't too complex or expensive to do might be most appropriate for someone in junior high. As others mentioned grading is done on the overall approach: selection of question asked, how well is it researched (internet has tons of good information readily available to look up), practicality of approach to -- including narrowing down a broad topic to something which can be investigated, documents the steps she's using--written and, if appropriate, photographic, and reasoning and then conclusions which she can support-- how she got there.
I am a scientist now retired but have judged science fair for years for a women in science organization I belong too. We give prizes to encourage young girls to keep an open mind about the sciences (not to succumb to peer pressure that it's not cool before they find out what it's all about). Our prizes are awarded to those who did not do the top projects but to those who showed promise and showed they cared even if they didn't produce something worthy of top awards.
I agree it does not have to be an experiment and very complex, especially if she's in junior high/middle school. As others have said working with living animals can be quite complex or even prohibited by the school. Check that and then have her consider if she has adequate facilities to keep the critters alive and happy. It's very difficult to get usable results if the test subjects die in the middle of the study. :)
Plants or seeds may be a better test subject.