Personalized 5th Birthday Mermaid Toddler Long Sleeve T-Shirt. Cute mermaid princess rides on her rainbow seahorse and surrounded by her fish friends. Happy birthday to the 5 year old on her 5th birthday. 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.