It is great that you are exposing your child to the wonders of science.
So I see your project as either the "effect of strawberry temperature on DNA extraction yield" or the "effect of ripening on DNA extraction yield."
You have the right idea of testing different condition to see how it affects the yield of DNA. Temperature is not going to change the amount of DNA in a strawberry, that is constant. But temperature could affect the extraction of the the DNA. If you read the link that Amy provided, you will see the procedures do not say if the strawberry should be chilled, room temperature or hot. You guys could be the first to determine if it makes a difference or not.
Ripening is a bit more advance of a topic and harder to control than temperature. It would be cool to see if ripening affects the yield of DNA extraction. Ripened fruit undergoes a lot of chemical changes that would affect the DNA yield, but most notably is that the ripened fruit will be less acidic.
Here is an interesting article for more reading on ripeninghttp://www.scientificamerican.com/artic ... t-ripening
Constant weight per trial:
You will need to run multiple trials for your experiment. In each trial you need to make certain that you are using the same amount of strawberry in each trial. This is especially important since you are testing the yield.
So tin a nutshell, this experiment is an investigation of conditions that yield more DNA.
Parting words: It is important that your child understands that the amount of DNA in a strawberry does not change from temp. or ripening. The gene expression do change in ripened fruit (which can be tested thru gel electrophoresis, but that is a bit more advance).