The best way for children to learn how to be more charitable, is for parents or authority figures to lead by example. children are observational learners, if you want your children to be charitable, you yourself must be charitable, so bring your children on your charitable endeavors, so that not only they can learn through your ways, it is also a great way of bonding and spending quality time with family by doing something objectively good. And another note, being charitable doesn't just mean giving material or monetary objects, it also means being kind and friendly to those you love and even strangers, or giving your time in helping others in need no matter how big or small, are also charitable causes that your children can learn from.