Oh snaps! A Christian question I'm qualified to answer! I'm not sure this will be five year old appropriate, because the cross is a difficult topic for theologians, but I'll do my best. This will be an oversimplification.
A lot of people think the people think the cross and forgiveness are about going to have a heaven when you die, but that's not really correct. The eternal life is more of a side effect.
The cross and resurrection are an invitation to partner together with God in the rescue of creation. Which means bringing the world back to the place God originally intended before humanity sinned.
In practical terms, it is the possibility of living like Jesus and treating others how he did. This way of living is so radical and transformative that not even death can overcome it.