Is it permissible to lie to one's parents by telling them that things will be bought with the money they provide, when in reality it will be given as charity, with the intention of increasing their good deeds?

Lying is forbidden and a sign of the hypocrites. However, it is permissible in specific situations to achieve a benefit or avert harm, such as: reconciling disputing parties, a husband and wife speaking to each other in a way that strengthens affection, and in warfare. What you mentioned is not an excuse for your lying to your family. You can combine their financial support for you with their charity to the needy by encouraging them to spend in the way of Allah without resorting to lying.