Try to keep their feet firmly on the ground. It'll prove easier in the end. Before approaching the celebrities, it is important to be clear of what they may end up doing. If your final requirements increase from the brief, the costs will certainly increase. Fees will rise.
Most celebrities have a few of their own 'pet' charities, which they support in their own way.
Linking celebrities to charities is not unusual, but can only work in the following way. The exception is when a speaker or artist is being engaged to perform at an event or dinner where tickets are being sold to pay for the performer or speaker. Asking a celebrity to donate his or her fee to charity is also not the greatest of ideas. Marketing sometimes requires mailing requests, questionnaires, invitations etc directly to celebrities. Fees are subject to requirement.