Third birthday princess party question?

My daughter is turning 3 and i'd like to throw her a princess party. the party will be mostly tinker bell but since i want it to a princess party as well id like to send out invations letting parents and children know that they can wear costumes if theyd like such as a princess,a prince knights and pirates. It will be summer though (august) so they dont have to go all out and wear heavy costumes by light costumes will do! how would i word that they can do so on the invation without writing a paragraph?