Re: Help me define my main objective for a specific lesson plan.

Based only on what you have described, I would recommend planning your 
scientific objective around the basic scientific process.  Loosely 
defined, the steps of the process are as follows:

1) Ask a solid scientific question.

2) Formulate a scientific hypothesis, capable of being proven wrong.

3) Devise an experiment to prove or disprove the hypothesis.

4) Perform the experiment, being careful to isolate dependent variables.

5) Evaluate the results of the experiment.

6) Develop a scientific conclusion from the results, with respect to the 
original hypothesis.

7) Start over.

For very young children, the details of this process might be a little 
much, but I have seen elementary school teachers break down such processes 
into flowcharts resembling board games so that children can see the 
progress through the process.

If you choose to pursue this direction, it’s important to stress the 
process, and not the conclusions, i.e. there is no win and lose, right and 
wrong.  When the process is followed correctly and ANY usable predictable 
results are found, everyone wins.

Also note that the scientific process can be applied to a large range of 
traditionally non-scientific topics such as cooking, computer programming, 
music, art, sports, etc.

Hope this helps!