@Neil702 I totally agree with you regarding that even if you have ticked the box not suitable for children, people can still instant book if they want to. Plus I must admit I think guests don't actually read the safety considerations, so it is difficult for them to see this. It should be placed more easily for guests to see
I think possibly this is so that Airbnb don't have to look as if they are discriminating against anyone.
I think if you have instant book on the only thing you can do is actually mention that you don't accept children and the reasons why in the description you do yourself about your property. Most parents would take your advice on board and search for somewhere which is more suitable and safer for their children.
Saying that we don't actually use the instant book feature for various reasons, one of them being this subject. We do have enquiries about bringing children and very rarely a request to book with guests bringing children.
Occasionally (i.e. if it is a baby) we do accept, but no to toddlers and infants as we have various features which could be dangerous. If we receive a request to book, we ask the guest to read the safety details and explain we can't accept and ask them to withdraw their request. We also mention that our insurance will not cover us for children, which is true. This has worked out fine so far.
Hope this helps