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Hello from JustAnswer. This is Dr. Love and I will be assisting you today.
Yes, the vitamin C should be taken daily along with the iron. This level of vitamin C is used to help increase the absorption of the iron. This is well above the level that would be needed to maintain good levels of vitamin C, so could easily be taken less often than daily from the perspective of the vitamin C. But to help increase the absorption of the iron, it would work better to take it daily with the iron.
This dose of vitamin C is also well below the recommended upper limit for a 12 year old of 1200 mg per day.
If she gets upper GI side effects from the iron, it will typically be apparent very quickly, within an hour or two. It is the local irritation of the GI tract that causes the nausea or vomiting. The lower GI side effects, such as constipation make take longer to be apparent, and may not be apparent for several days.
If I can provide any clarification, please let me know.