I LOVE this idea!
Young children have a hard time understanding the concept of self-sacrifice much less understanding what Jesus sacrificed for them. They don't know how to self-reflect; it's a skill that develops as they grow. They are learning to share and what it means to be nice to each other and Lent is a great time for remembering how much we have and sharing it. They also love putting money in the offering plate so why not try having a Children's Change for Charity jar throughout the Lenten season.
During Lent have the Sunday School kids decide on a local charity they want to donate their Lent offering to (give them 2 or 3 choices) and create a jar that can sit near the front of the sanctuary with a huge label naming the charity on it. During the offering let the kids go up and drop their coins in the jar while the ushers pass the plate. At first the very young may need to be led but soon they'll go on their own when they see the other kids doing it. If the change makes noise as it drops in the jar, let it. Kids like that!
On Easter Sunday (or the Sunday after) make sure you announce where the money is going and if possible, invite somebody from the organization to be there to thank the kids. If not, be sure to let the congregation and kids know how their money will help others. Use pictures so the little ones can better understand how their change can change somebody's life! We love because He first loved us! 1 John 4:19
This is something that can be done all year long but would definitely be a cool Lent tradition!