Easiest would be the idea of having several talks, presentations, or workshops. To really get the benefit of having a mathematician come in we would need to concentrate on non-curricular material, and every party would need to agree that that would be actively desirable. We could say that connections to the curriculum would be desirable, but not essential, allowing that we are looking to inspire and enthuse, rather than actively train the children.

And not to leave out the teachers. We'd want to have sessions for them to talk about the maths, and to air questions and concerns about the material they teach. In short, they also need to take advantage of the opportunity offered. This can be formal, or informal. Simply having someone around who will talk about maths, rather than teaching maths, might be enough.

These are the questions. I think the right person can make it work.

What would be regarded as a success? There I'm not so sure. Should there be a package of material left behind, and that count as part of "the deliverable"? If so, what should be in it? I think that must be driven by the teachers. This is not material for teaching, but it is material for the teachers.