Some countries have legal requirements for Instructors working with children under 18 years of age.

Below is a few countries with requirements. 

Australia

Each state is different.   Application is made direct with the government Department.

NSW:  Working with Children Check.  
QLD:   Blue Card.   
WA:   Working with Children Check.  
VIC:  Working with Children Check.   
SA: Screening Checks / Criminal History Check.  
TAS: Working with Children Registration.   
NT:  Working with Children Notice.  

United Kingdom

England & Wales:   Enhanced Disclosure and Barring Service (DBS) checks.  (Previously call CRB). Enhanced check required.    

ASI is a Registered Body and can organise Disclosure checks.  Please email us for information.

Scotland:   PVG (Protecting Vulnerable Groups)  / Scheme record check

Northern Ireland:  Criminal Disclosure Check 

Europe

Ireland 
Garda Vetting.  
Code of Ethics & Good Practice for Children's Sport   
Children First   

Other Countries

Not all countries have requirements.
Always check in the country you are working if they have requirements.