Find your school nurses contact details. Work with them to plan sessions that you think require a more experienced hand than you can delive. Don't ask them to come in and teach x without meeting before, discussing the aims, children's needs, teaching approaches they'll use and planning sessions together. Give feedback after too..I loved this bit, maybe next time we should do so and so to include Jonnie?
Chances are they have access to some artefacts you don't too.
I've invited nurses, paramedics, religious people, councillors, MPs etc to help give a specialised viewpoint on the topic at hand. Contact them well in advance and ensure you meet before hand to discuss expectations and plan the session.