Build a Jenga tower. This is like the hard heartedness we build up when we are hurt by someone and can't forgive them. It feels like a solid mass we can't break down easily.
Slowly start to take bricks away- children could take it in turns to do this. As each brick is taken away, pray 'Lord help me to forgive'. This symbolises the fact that we can't break the tower of unforgiveness ourselves, we need to ask God to help us with it and, bit by bit, give the hurt to God. We might need the support and help of others too.
Eventually the tower will collapse. This shows that, even though it takes quite a while, if we keep asking God to help us to forgive, our hard heartedness will collapse. The bricks remain to show that what happened still happened but God has now rearranged how we feel and broken down the hurt.