"The children eat the large laulau."
Translation:ʻAi nā keiki i ka laulau nui.
In Hawaiian, we "announce" everything and everyone. Nā indicates/announces a plural of a noun: nā keiki (the children). Ke and ka indicate/announce a singular noun: ke keiki (the child), and ka pua (the flower).
When it comes to names, the announcers are ʻo or ē. Each one is used differently, depending on how we address the person. To address someone indirectly (or talk about them) we use ʻo. ʻAi ʻo Kaleo i ka laulau nui = Kaleo ate the large laulau. (We're talking about Kaleo). When we address someone directly, we use ē. Ē Kaleo, pehea ʻoe? = Hey Kaleo, how are you? (We're talking directly to Kaleo).
So, to answer your question, we donʻt use ʻo because keiki is not a personʻs name (in this instance) and there is already an announcer in the sentence, nā. Coincidently, if the personʻs name was Keiki, and he or she ate the large laulau, you could say, ʻAi ʻo Keiki i ka laulau nui. ;-)
Bonus: How to tell whether you should use ke or ka (in most cases). Nā is the plural indicator for all nouns. When it comes to singular nouns, though, the announcer may be ke or ka. In most cases, you may use the KEAO rule = if it starts with K, E, A, or O, (or with a kahakō version of the vowel) then ke is mostly likely the announcer. If the word doesnʻt start with K, E, A or O, (or starts with an ʻokina) then use ka. There are some exceptions: ke pākaukau (the table), among others.