The official latinized name of the show is Märchen Mädchen, but as the character ä can not be used in danbooru tag, currently the tag in danbooru is marchen_madchen. However it seems like the general transcription rule as well as official material for the series prefer to romanize the ä as ae, as reflected by for example official publications, official site domain name, and official twitter account user name, therefore it seems like maerchen_maedchen would be a better name for the tag.