No. Fanfiction that involves sexual content is not appropriate. It goes against moral and ethical standards in most cases, and may also violate the rules of certain platforms or communities.
One way is to use reliable fanfiction platforms that have strict content moderation policies. They usually screen out works with inappropriate content.
One common theme is adventure. Since they are often portrayed in a fantasy or medieval - like setting, going on quests and exploring new lands is quite common.
A storyline could involve the baby getting lost in a magical world. Arthur and Selenia then have to embark on a quest to find their baby. Along the way, they meet various interesting characters who either help or hinder their search. This would add an element of adventure to the fanfiction. It could also show how their love for their baby drives them to overcome all obstacles in their path.
Love at first sight is also a possible theme. They see each other and immediately feel a strong attraction, which leads to their kiss. This can be developed in different ways in the fanfiction, such as exploring how they deal with this sudden and intense feeling.
You can try searching on popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you might be able to find some great arthur and selenia kiss fanfiction there.
The main characters are Arthur and Selenia. Arthur is often portrayed as a brave and adventurous character. Selenia, on the other hand, is typically a character with magical abilities and a strong will.
Arthur could develop by becoming more responsible. With a baby in the picture, he has to step up and be more reliable. He might learn new skills like changing diapers or cooking baby food. Selenia could develop her nurturing side even further. She could become more patient and understanding, especially when dealing with the baby's tantrums or sleepless nights.
Another fanfic could be a love story between them. Maybe they start off as friends but gradually realize their deeper feelings. They could go through some challenges, like differences in their social status in their respective kingdoms, but in the end, their love overcomes all. In this kind of fanfic, there would be a lot of sweet and tender moments between them.
A plot could involve Arthur and Selenia attending a grand festival in their kingdom. There, they participate in different contests. However, they start to face some jealousy from other characters. They have to deal with this while also enjoying the festival. As they do so, they realize how important they are to each other and their feelings for one another start to change in a more romantic way.