You will have the disney dining plan for each NIGHT of your stay. So if you are staying 7 days, 6 nights you will have 6 snacks, CS and TS for each person in your party. You can use them anytime that you want to. There was a day that we only did TS and a day we only did CS. Whatever works best for you!

I know with CS this is not an issue and I know with family style or buffet we would need to pay for her child's meal - I'm concerned about places like Concourse, Teppanyaki, Mama Melrose's, Sci Fi, & Whispering Canyon (we won't be getting the skillet but ala carte meals instead). Maybe it just depends on the server and maybe if we don't get an agreeable one we can just have it adjusted later, especially if she doesn't order anything.
HELP!! 
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It seems to me the pineapple is part of the meal and should not be considered dessert. At Ohana they take great pride in their desserts and usually show you a sample of them as you enter the restaurant. I really doubt there would be any problem ordering your own desserts using the plan. You could always call the restaurant manager and ask but again, I doubt this is the case.