I'd have thought people would be able to accept that whilst they'd like their children to be invited, people choose not to invite them for whatever reason - that might be numbers and cost which mean children are limited,mor it might be because they just prefer an adult only or predominantly adult only event.
I think we'd agree the day is about the bride and groom so their choice. I can totally see why for practical reasons people with kids might choose not to go, due to difficulty of arranging child care, cost etc. I think those getting married understand that.
What I struggle to understand is when people refuse to go because they are offended that their kids haven't been invited and take it as some kind of sleight on themselves or on their overall family, when practicalities mean they could go. Even if some children are invited (because the bride and groom feel closer to them) and yours aren't, it isn't a great offense to you and your family. And as lomg as practical reasons don't prevent it, it's perfectly possible to have an adult day out.
I actually think that often when people turn the invitation down for non-practical reasons, they WANT to make a bit of a scene and show they are offended. They want to make it about them and their invitation not the bride and grooms weddi g.
People turn down wedding invitations for all kinds of reasons. There are often practical reasons or occasionally because they don't like the people. Even when the bride and groom have invited people to come a long or to do something inconvenient such as soending a lot or leaving their children, most people manage to decline without feeling a need to go into things about the invitation that they object to or offend them, but just decline politely. Some people are determined to make a battle or fight out of it and worsen the relationship and make it about themselves. They reall Y want the bride and groom to realise they have caused offence or upset.
In this scenario Op, fine don't go if you do t want to have an adult day out. Fine if it's not practical or you aren't interested in these people....but try to move beyond feeling cross that some children were invited and yours haven't been. And definitely resist the urge to say this is why you a rent coming even if you put it in faintly veiled terms. Rise above that.
And definitely don't talk to your kids about the mean relatives leaving them out. That really isn't going to help anyone. Instead use it as an opportunity to simply explain that often people can't invite everyone to parties and it's not an offence to them.....they will have many times in life where they're not invited to things and it's good to get to grips with this sooner rather than later and not take offence. But unfortunately parents who easily take offence usually teach their children to do the same too.