@Bert-and-Marianne0 You make it extremely clear in your listings that you don't accept children, so I think you're doing everything you can do to draw people's attention to it.
If you are still getting these bookings, I would ask the guest to cancel their reservation. If they won't, I would contact Airbnb and ask them to cancel for you penalty-free: you are allowed to cancel Instant bookings without penalty if the guest states that they are going to break one of your rules. Just make sure you do it through an Airbnb agent so you don't get dinged.
If it's just an inquiry, just send the guest a reply stating that you don't accept children as guests. If it's a booking request, decline and state that they're going to break your rules on your reason for declining.
I agree that it's really strange and annoying that the Airbnb site allows guests to make bookings that directly break your rules. Surely there should at least be a warning that says something like "We see that you are bringing children with you on your trip. This host doesn't allow children. Here are some other properties that do that you might want to consider."