I saw a warning in Japanese on a swimming circle, and it felt like something was wrong. I’m asking for advice from an expert.

Your understanding of 子公の人で游ぶのはriskないです is indeed correct, and the exact grammatical requirements are the same.

However, the slogan on the lifebuoy is also the most Standard Japanese expressions

....は.....です

人游びは noun as subject

risk ないです adjective and auxiliary verb are formed together Predicate

Check to see if your grammar book says so.

Besides, slogans are generally required to be concise and clear. The original sentence of the poster is correct. If it is in a slogan, people will usually use the simplest one