Would you allow your children to be left alone with someone you haven't met? (See details)

My ex-husband has a new girlfriend with twin 13 year-old boys. These boys have been left alone with my sons (ages 11, 10 and 7), taken them to the park, and are planning to go camping with them. I have never met these boys, I know 13 can be a challenging age and I am nervous about the impression their behavior might have on my kids – especially the 11 year-old. I have asked the ex to set up a time for me to meet these young men, but he thinks I am just trying to make trouble for him. What do you think? Would you want to meet these kids? FYI, the ex isn’t a bad guy, but I can honestly not say that I have a ton of faith in his ability to assess people’s motives and behaviors.