On my tree an extra data box has appeared alongside our children. We are well past the age to have any more children. Can anyone suggest a way to remove this.?
I assume the "extra data box" is labelled "Add Child". This cannot be removed, in the same way that the "Add Spouse" box is a static feature, regardless of whether the individual died in childhood, or is known to have never married.
There have been requests in the past for the ability to remove these boxes, especially the "Add Spouse" one, when facts are clearly known - e.g. individual could not have married or, in your case, a woman is past child-bearing age. However, there appear to be no plans by FamilySearch to implement these suggestions.
Just think of the situation as being similar to a form where many of the sections are not relevant to you. Only, in the case of Family Tree, you cannot enter "Not applicable" in those boxes. Perhaps you might wish to add a Note in the Collaboration section(s) explaining, with the reason, the children listed below you are the only ones you have.
I like what Paul suggested, "Just think of the situation as being similar to a form where many of the sections are not relevant to you". There is another option to consider.
There is a box on the person page for every individual in your tree called "Other Information". Within that there are a number of pre-loaded categories of information that you can check to add a comment about the person whose page you are on. There are also categories for adding a custom fact or event which you can use to explain whatever may be relevant.
These comments are visible to all who come to the page.
I hope that helps answer your question.