Vision testing in preschool children

Hello
I am an orthoptics student. I have an assessment on assessing ion in preschoolers in which I'm struggling with. Could someone please help if you guys have any ideas/suggestion ??

What are some of the challenges that may be associated with the examination of a preschool aged (4 year old) child’s visual acuity? How might you modify your communication, behaviour and testing method to cope with these challenges?

Thank you