Lauryn,
allow plenty of time for this activity, it is one we are very familiar with at Blackrock Castle, and it takes time for the children to work out where best to place the straw and how little tape is needed to attach it to the balloon.
If the straw is a bit off centre, the balloon whizzes in a circle rather than gliding elegantly along the string. We’ve also discovered that fishing line is the best string for us. It is smooth and strong and can be pulled very taut.