Answer:
reading stories
Explanation:
Parallel structure is when you use the same pattern of words in different phrases of the sentence in order to make it more fluent and give te same importance to all the parts of the sentence, you can do this by making the different verbs of the sentence gerunds, in this case you can see a parallel structure already built in the first two verbs
"the siblings enjoyed trapping fireflies and collecting seashells more than"
With this you can see that the next verb should also be a gerund:
"the siblings enjoyed trapping fireflies and collecting seashells more than reading stories"
