Is swimming allowed there? my grandson want to go with his friends who are going there, but it looks dangerous to me. Any advice ??

This is private property and is marked "No Trespassing". Those caught trespassing are charged by the local police. See the recent article by the local newspaper. https://www.thepilot.com/news/sheriffs-office-issues-nearly-100-citations-for-trespassing-at-robbins-rock-quarry/article_8668735c-a05f-11ea-aaf9-5ba8ce96168f.html

It is allowed