Most often it is not, unless required by the order. However, it has irrefutable benefit in proving and keeping up with your own competence and understanding.
Operators are responsible for the safety of playgrounds and recreational sports areas by law (details may vary from country to country, but as a general principle this is true). Ignorance doesn’t free one from responsibility, but in the case of an accident it can make consequences even more serious.
In conclusion: No, certification is not mandatory, but it is a great tool for assuring a high quality of operation, as well as for reinforcing your own position in case an accident occurs.