personally, I think competitions at the second grade level are inappropriate. However, you child might try this:
Shallow dishes with rim (ideal: circular Petri dish)
Food colors (all 4)
Toothpick or coffee stirrers
Liquid dish detergent
Pour the milk in the dish to a level of about half a cm
Invite students to see the dish as a clockface
Place one drop of each different color around the “clock” at 12, 2, 6, and 9:00 o’clock positions.
Swab the bottom of the toothpick in the detergent and then stick the toothpick into the center of the “clock,” touching the bottom and then lifting and holding the stick suspended in the milk
Observe what happens
Invite students to move the toothpick up and down to create additional movement of the colors.
The detergent emulsifies the fat in the milk (4%), setting up a convection motion in the liquid that then carries the food coloring along with the fat toward the detergent.
To make this an "experiment" : compare with skim or 1% milk.
There are many science fair projects that you can enjoy doing with your second grader. I also have a second grader and below are some projects that I know he will enjoy doing.
1. Can magnetic attract through paper? (The data you collect would be the thickness of paper starting with tissue paper to card stock paper)
2. What is the temperature and melting rate for: ice, ice cream, whip cream, butter, chocolate?
3. What is the effect of salt on ice?
4. How do icicles form? How do hails form? How do dews form? How do twister forms? (Each of the questions in #4 is a separate experiment. You can find the instructions on these experiments at the link below)
Below is a link to science fair projects for elementary school students. Have fun!