Punnett squares are a great way for children to visualize the way dominant and recessive traits work and how they appear in children or offspring. Using Punnett squares to replicate the foundational pea study of Gregor Mendel is a great way to introduce genetics and Punnett squares to children. Use Mendel's genotypes of TT, Tt and tt in different combinations on Punnett squares. Have children determine all the possible genotypes from those two "parents" and how many will be T - tall, and how many will be t-short in phenotype, or the way they look.
Older students are able to understand that not every cross of two "parents" will yield exactly four offspring. Instead, the Punnett Square shows the expected ratio of offspring with a particular genotype. A coin toss activity will show more accurately the way this works in nature. Students work in teams, with each student having a coin. Create a chart listing four "families" in the rows. The columns list the possible genotypes, such as Aa, AA and aa. Heads is assigned one trait, say the dominant trait, then tails is assigned the recessive trait. Have students flip the coin and fill in a chart based on the results of the coin flip, marking a tally mark under the appropriate genotype. Have the students determine the percentages for their own small sample and see if they match expectations from the Punnett square. They will likely be off in individual groups, but when data from the entire class or several classes is combined, the results are usually very close to the predicted outcomes from a Punnett square.
Have students look at simple dominant and recessive traits found in humans and create Punnett squares to determine genotypes in their own family and see simple examples of dominant or recessive traits. Many characteristics, such as eye color and height, are determined by numerous genes. However, there are a number of human traits with simple dominance that make good examples. For instance, the ability to roll the tongue into a circle is a dominant trait. Children can survey their parents, grandparents and any siblings to determine the genotype of their parents for tongue rolling, then create a Punnett square to determine the likelihood of any children being able to roll their tongue. Another trait with simple dominance is the bent back "hitchiker's" thumb (straight dominant, curved recessive) and dimples (dimples dominant, no dimples is recessive).
Another way to create Punnett square activities is to use cartoon characters with distinctive characteristics and imagine dominant and recessive genes that cause those characteristics. They can then create Punnett squares to determine characteristics of possible offspring. For example, sponges such as SpongeBob could be dominant for the square shape, while round is the recessive trait. Tiggers may be dominant for pink noses, with black being recessive. Bart Simpson may have a dominant gene for a spiky head. Either assign a genotype, or give students a scenario that requires them to figure out a genotype.