Holy Week is actually really well suited for kids - the whole week is full of tangible symbols and stories that reach to the heart of Christian faith.
Honesty and simplicity are key:
Why did Jesus die? Well, the leaders of his day were scared of him and thought he was dangerous - even though he was not violent at all.
Why did he die on a cross? Crucifixion is how they killed people leading rebellions - so that the person was really visible as they were dying and their followers would get scared off.
Why do we call it "Good Friday"? Well, though Jesus dying is really sad, people ever since have seen God's love at work in his death, showing how God is with us even in sad times - even with us in death. As well, because Jesus said, "Father forgive them, they don't know what they are doing" people have learnt that God can forgive even the most terrible things. That God loves us so much that there is always a second chance for us.
You might also want to talk simply before services:
Some lovely traditions have grown up around Holy Week that are very accessible to kids. Note - most of them have to do with food!
The file is attached from our fellow Diocese of New Westminister in BC: