The scientific method states that your daughter should come up with a hypothesis and then test against that hypothesis with a (repeatable) experiment.
If you are in a cold climate, you could try experimenting with freezing temperatures. You could test which cups are better insulators: which would freeze last. See if she can come up with a theory as to which would freeze first, etc... Then have her place them outside and filled with the same amount of water, and check them every 5minutes and record the progress. Once they are all frozen, she can compare the results with what she expected, and what conclusions can be drawn.
I've also included a link to a similar experiment using ice and salt.
Your hypothesis is "Do cut flowers last longer if you put them in cold water or warm water?" This is a great one for a 7 year old. Simple but teaches biology. This is a nice experiment because it uses carnations and food coloring.
A good and fun project for someone that young is to see if people can guess the smell of something with their nose plugged. You test them before and after being plugged and see what the answer is. It is simple and they can see the methods used.
My daughter had her first science fair project ! Due to the fact that she is just being introduced to science this year I went to barnes and noble and bought her a box of different experiments! It will help explain the project to your child ! Try not to over complicate the project ! Remember that they are only 7 or 8 years old ! Good luck!