Personalized Mermaid 4th Birthday Toddler Long Sleeve T-Shirt. Mermaid 4th birthday! A cute little mermaid celebrates your fourth birthday with her fish friends. Cute gift for the four year old. A classic for Fall, Winter and Spring, these long sleeve toddler t-shirts are sure to be a staple in your child's wardrobe. Whether layered or alone, this t-shirt is soft yet durable enough to stand up to your toddler's playtime demands.