"The children camp in the spring."
Translation:Hoʻomoana nā keiki i ke kau kupulau.
I do not fully understand this rule yet, but the answer to your question may have to do with the word (Hoʻomoana / camp).
Nā ʻAmi Lauka / The Lauka Prepositon: (i) and (iā)
The lauka prepositon (i or iā) occurs after actions that are directed toward, affect, or refers to something else. For example, reading (heluhelu) can be directed towards a book, eating (ʻai) affects food, and seeing (ʻike) can refer to a house.
Take a look at the slides in the link and see what you think:
The problem exists in other sentences with other verbs. But I don't think that would change your answer. Most verbs directly effect some noun which should be marked in this way. But, I don't think that applies here. That is referring to marking the object directly affected by the verb. The seasons are not the object of the verb, but instead are a type of time stamp. If you are saying the hour at which something occurs, this course accepts either i or ma, but if you are saying the season in which something occurs, this course only seems to accept i.