It comes down to whether you want to see your friends more than you don’t like accommodating their offspring, I suppose. If you say ‘Absolutely no children’ on the invitation, it will rule out anyone who is still breastfeeding, and anyone who can’t get someone else to look after their child, so the event won’t be the same, anyway, if only the childfree attend.
Realistically, annual events like these change over the years anyway, as people’s needs change — not just whether they’re parents or not.
I go to an arts festival a long drive and a ferry ride away annually, car-sharing with friends, and have for a decade or so. Which ferry we get there and back, and whether we stay over has fluctuated down the years, depending on who has no childcare, who brings a child or children with them, who has some childcare but need to get back by a certain time, who has an ailing dog, who has other caring responsibilities etc.
But, however mildly frustrating some of these restrictions/interruptions etc have been at times, I don’t think anyone wants to say ‘Look, don’t come at all unless you can leave your children with someone else for an open-ended period, because you’re wrecking the vibe if we have to get the 4.30 ferry home because you’re on the clock or little Irma bawls at a poetry reading and has to be taken out’. We’re all friends because we fundamentally like one another, and are prepared to be flexible according to one another’s needs at a particular time. I’m fairly sure we’ll all still be friends when all the children have left home.