Kids

I would have a no kids wedding and try to incorporate something that might keep your son entertained. No one would have an issue with your son being the only child, especially since he'll be a part of the wedding. Maybe have an activity he can do during the reception?

If any of your friends have newborns, I would assume they will want to bring them regardless if kids are invited or not just because that's pretty typical. Do a lot of your friends have kids? I guess it will just limit who all comes since sometimes it's hard for people to find sitters.

A 12 year old who is participating in the wedding is different than a toddler who is only going to be attending because of babysitter issues.

I honestly don't think you need to explain why you're allowing your son to attend the wedding and not other children. Be aware that some parents won't come just because it's too much of a hassle to attempt to find a babysitter. Just don't make exceptions for one family and not the other...that's where I think you'd get into some trouble with people.

As for entertaining your son...I don't really think that will be necessary. Dancing and food alone will most likely be enough but you know your son better than I do. If you think that he'll need to be entertained than you could probably give him an activity to do like a picture scavenger hunt where he takes pictures of people and things from a list you give.

He's your son. I'd give him a plus 1 or 2 and if anyone has the nerve to question or complain about it, I'd stare silently then walk away.

I'm in the same boat with like 2 of my cousins. So we set the age limit to the age of the youngest "included cousin". You may end up with a couple of kids above that limit aside from your son, but it's fair. OR since it's or wedding, nobody will be upset that your own son is there. But for him being bored (I say this from experience), I'd just bring his favorite activities and designate of of the grandparents as a pseudo babysitter.