Hannah's hands reached out slightly when she saw Ron yelp, just in case. While she did not think that he would actually drop Lizzie, there was always a chance something could happen and Hannah naturally felt the urge to create a barrier if one might be needed. Getting hair pulled unexpectedly was perfectly reasonable to jolt, after all.
"Are you sure you don't want me to take her back?" Hannah asked, reaching over and prying the few hairs out of Lizzie's hand. "I'm sorry, it's just the age." she apologized quickly, explaining out of habit.