Along the lines of keeping it fun, pressure is not fun for anyone learning something new.
This is just as true for kids as it is for adults.
If your student flubs a cast, help them brush it off by encouraging them to try again or suggest a tip to help on the next one. If their fly lands in the tree for the fifth time, make suggestions on how to amend what they are doing, smile and move on.
While it’s fun for experienced anglers to hook into a fish on the first cast when reaching the river or after changing a fly, that expectation for a newbie can be intense. Set the expectation early that this outing is all about having fun, exploring, learning and improving skills.
Laugh and smile when things go wrong and your student will learn that all this is not that big of deal and it should be fun, always.