There are two common mermaid tail designs. One is with an open bottom without a monofin that children can easily wear like a skirt. They can either leave the bottom open so that they can walk on the beach or close it for an ultimate dressing-up experience.
The other design is with a closed bottom where a monofin can be inserted. Small children may find this design difficult to wear without assistance. To do so, insert the monofin in the tail (bottom) part then let your kid's feet slip into the pockets. Once both feet are in, pull the tail up to the waist.