To give your mermaid tail structure and swim like a real mermaid, you will need to insert a monofin.
Children (with adult supervision) can swim faster and more efficiently using what is known as the 'dolphin kick', using the monofin.
These are lightweight and particularly comfortable to wear (we find the plastic or rubber ones can pinch the skin). The central plastic fin is covered in soft, flexible neoprene material that cocoons the ankles.
We stock two sizes of monofins and up to six sizes of mermaid tails. The tail size 130 (suitable for approximately 7-8 year olds) sits somewhat in the middle of the two monofin sizes.
If you child has size 1 or above feet, we would recommend getting the larger size as it is important that in the water they can quick release their feet if need be (please read safety advice).
Also, once you have the larger one, you won't need to replace it, just update the mermaid tail as she grows!