Dictating the types of gifts you will receive is a borderline issue in terms of etiquette... When the gifts will be copious and expensive, such as a wedding, quinceañera, confirmation, a wedding anniversary that is a multiple of ten, a bar/bat mitvah, and a school graduation. In those cases the hostess can make a statement such as "Jane has requested that she receive donations to her college fund in lieu of gifts" or "Jane and John have registered their preferences for gifts with Gumphrey's Department Store of Peabody, Ontario."
For a relatively non-important event, such as the ordinary birthday of a child between 2 and 17, the only option you have to impose your preferences on those who wish to honor the event is to redirect gifts to a charity: "Please send donations to XXX charity in lieu of gifts" noted with the invitation. Otherwise, telling your guests what to give is considered to be a breach of etiquette. The point of the event is to allow the guests to express their love and regards to the guest of honor in a way that honors their natural affection. To dictate how they do that is impolite and would diminish the opportunities for unique and meaningful bonds between the guest of honor and those honoring him/her.
(An announcement of the event should be sent *after* it occurs and cannot include a reference to expected gifts. Only persons invited to an event can receive directions about the gifts they will bring.)