Getting children excited about giving money to charity is actually pretty easy. They have an innate sense of the value of helping others, and a sympathy that is often erased in adulthood. They don't stop to question whether the homeless person at their door is deserving or undeserving - they think it's sad that he or she is homeless and want them to have a home.
Lent is a time to bring those issues to the forefront of our life together. Lent is when we look at our sins, individually and collectively, and seek to reach out to others, setting aside the parts of our life that put walls between us and God, and between us and Christ in others.
Download Christian Aid's Lenten calendar, which can be found here. The calendar has a fact about world poverty for each day of Lent, and connects that fact to a way of giving money - for example, for Ash Wednesday, the calendar says, "only 15% of the world's population lives in a country where there is freedom of the press. Give 10p for every newspaper you read today." Many of these devotions will need some explaining to children, so this is something for parents and children to do together.
Then get some plain cardboard money boxes, which you can purchase from Baker Ross. (And yes, they do next-day shipping, so you can get them before Sunday!) Children can decorate their own money box and use it to collect their contributions during Lent, bringing them in on Easter Sunday.
We're doing this in Sunday School this Sunday - hopefully I'll remember to bring the camera and get some photos!