Get the organization you are raising money to agree to a Tom Thumb Wedding and then you should procure a copy of the play. Next you have to decide how you will cast, whether it will be an audition process or you will assign the roles. The leads should definitely be children. The director can choose whether to cast the rest of the play with adults or children.
Put together the location while the director works with the actors. If you are doing this through a school or church, you might already have a venue. If you do not have a venue that is the next step, secure a location for the play.
Re-create the original invitations. If you have a mailing list, you can choose to send a direct mailing to all those you would like to attend, including the price of the event. If you do not have a mailing list you can choose to put up fliers or take out ads in the newspapers.
Issue invitations as people purchase their tickets. The invitation will serve as their entrance to the play. Most people hand out the cards with the images of Tom Thumb and Lavinia Warren as the actual tickets, once they are paid for. Consider whether you will offer a "will call" option so people might pay when they arrive at the show.
Decide how you will be collecting the money. If you are having children, sell the tickets or other parents give specific directions in regards to accepting cash, checks and credit cards.
Work on putting together the costumes and set for the "nuptials." This should be done concurrently with getting the tickets sold. The nice thing about the play being a wedding is decorations can be as limited as you would like with simple flowers and candles. For the costumes, thrift shops are often a good location to track down vintage-looking clothing. You can also rent from a costume shop.
Put on the play. You might choose to do multiple performances or focus all your energies on a single event.