As others have said, the love bombing is the biggest thing to be aware of your first time out. If you're aware of what's going to happen, though, it will give you the objectivity to not be sucseptible to it. You'll likely quickly understand how someone in an emotionally vulnerable state could get pulled in.
I would also reccomend that you read this information on the BITE model just so that you can be on the lookout for ways in which they are able to control people. You'll almost certainly see examples of emotional manipulation at every meeting, and the behavioral control will be obvious when you walk in and see a bunch of clean-shaven men in suits and women in dresses (no pants) etc. The information/thought control is a little less frequent but it wouldn't surprise me if you caught some good examples of that too.
Also, while far from the magic mind control ray that you fear, I would also suggest that you not participate in singing the songs (someone will almost certainly offer you a song book), or bowing your head during the prayers. There's a known psychological effect that occurs when you perform small acts of conformity like that and it will cause you to relate to the cultists a little more and you'll lose some of your objectivity. I don't think it's likely that you'll get sucked into the cult, but it's important to stay as detached as possible so that you can recognize and call them on their bulls**t if needed.
All that is probably overkill, but I'm the type that prefers to be over-prepared for things and figured if you asked the question you might be too.
What would you suggest? I have no idea other than to walk in and act like a deaf mute....
It depends on what you're hoping to gain by the visit, but I don't see any reason why the truth (that your son joined up and you thought it was time that you saw what it was all about) would hurt anything.