Take frozen bread dough (you could make bread from scratch - but frozen saves time ;-)), thaw, and twist it into a braid and then into a circle making sure the ends are together. After it has risen the last time, have your children place sharp round toothpicks all over the braid. Then bake. Don't stick the toothpicks in too far as you want it to look like a crown with thorns sticking out.
As your children are caught being good, carrying out a work of mercy, or other good works, they are allowed to pull a thorn out of Jesus' crown and place it in their jar. (Each child has their own jar). Remind them that every time we do these good things we are truly helping Him but when we do the opposite we are wounding Him again so children usually try a little harder so as to be able to remove "thorns" from His "crown".
At the end of Lent, count the "thorns" in each your children's jars. You can decide how much money to give them for each thorn. For example.....a dime for each thorn. 10% of their total could go to the support of the parish and they can then be encouraged to use the rest for a good purpose such as sending a portion of it for rosaries for those who do not have them in foreign missions. But they could also be allowed to use it as they decide.