Read 2 Corinthians 5:17 and talk about how Jesus would not be able to make even one mark on His egg, because He never did anything wrong; He never sinned.
Have your grandchildren take water and wash their egg - the marks should wash away. Talk about how when we believe and ask Him for forgiveness, Jesus' death and resurrection takes away our sins - they are gone! Lead your grandchildren in thanking Jesus for His great love and grace!
Handing down the faith does not have to be hard - it can even be fun!