What are the risks and likelyhood of each risk for spinal tap on 18m old?

Depends: This depends on the experience and proficiency of the person who does the tap.In experienced hands the risks are negligable. I never had any problems beyond localized bleeding/failure to gain the right fluid in thousands I did over 4 decades.Most permits list possibility of infection or nerve injury, but this is more speculation and desire to be inclusive on the form.