Chef- This question is more applicable to the Grades 6-8: Life, Earth, and Social Sciences but I can try to help you.
This sounds like an interesting project. A quick Google search of "what affects heart rate in daphnia" will help answer many of your questions. There are numerous online sources and videos about Daphnia and heartrate. Here are 2 very quick sources about caffeine and Daphnia.
http://en.wikipedia.org/wiki/Daphnia
http://en.wikipedia.org/wiki/Caffeine
Here is a list of common caffeine amounts in beverages. You can use caffeine pills such a No-doz and Vivarine (their caffeine content will be listed on the box), additionally you can use different beverages that contain caffeine (soda, energy drinks, coffee). Remember to be sure to have a control beverage or 2 such a water, that does not contain caffeine. You will also want to note the other ingredients that are in the beverages.
http://www.cspinet.org/new/cafchart.htm
The Science Buddies Website is also always resource
https://www.sciencebuddies.org/science- ... p048.shtml