One of the great things about homeschooling is that you can incorporate your school lessons with your kids' spiritual lessons. I came across this great activity in the Focus on the Family, Thriving Family magazine last year, that is perfect for discussing Jesus' perfection and how he took our sin upon himself. It also works great as a science lesson discussing plants. It is relatively simple, all you need is a few white carnations and some red food colouring. For me, the less materials the better, so I think it's a perfect activity!
First read 1 Peter 1:18-19,
The blood of Christ set you free from an empty way of life. That way of life was handed down to you by your own people long ago. You know that you were not bought with things that can pass away, like silver or gold. Instead, you were bought by the priceless blood of Christ. He is a perfect lamb. He doesn’t have any flaws at all.
Discuss with your kids how Christ was blameless and how we are sinners, talk about the sacrifice God made for us. Then have your children examine the white carnations and look at how white they are and compare them to Christ's perfection. When you put them in water you need to add a fair bit of read food colouring to the water. In a few days you will see the perfectly white petals turn pinkish/red. Then discuss with your kids that this is what happened when Christ died on the cross, he took our sins so that we may have a relationship with Christ and look forward to the day where we will enter Heaven. This experiment becomes a great visual demonstration for the kids.
If you are interested in taking the discussion into a science discussion afterwards, this is a great link that explains the experiment and gives you some explanation as to why it works: http://www.kidspace.com/index.php/2010/experiments/colorful-carnations/