**Core Concept**
The question is testing the knowledge of a specific type of dental stain that occurs in children, particularly associated with the eruption of new teeth. This condition is related to the consumption of certain foods and drinks that contain chromogens, which can bind to the newly erupted tooth enamel.

**Why the Correct Answer is Right**
The green stains on the child's teeth are most likely due to the consumption of **fluorinated water or fluoride supplements**. Fluoride can combine with other substances in the mouth to form fluorapatite, which can give teeth a greenish discoloration. This is a common phenomenon in children who consume fluorinated water or take fluoride supplements, especially during the eruption of new teeth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the condition described in the question. While **tetracycline** can cause tooth discoloration, it is typically associated with a yellow or brown stain, not green.

**Option B:** **Erythema multiforme** is a skin condition that can be triggered by various factors, including infections and medications. It is not related to dental stains or the consumption of certain foods and drinks.

**Option C:** **Amalgam tattoos** occur when mercury from dental amalgam fillings is released into the surrounding tissue and causes a blue-gray discoloration. This is not associated with green stains on newly erupted teeth.

**Clinical Pearl / High-Yield Fact**
It's essential to note that excessive fluoride consumption in children can lead to fluorosis, which can cause white or brown spots on the teeth, in addition to green stains. Parents should be advised to monitor their child's fluoride intake and consult their pediatrician or dentist if they have concerns.

**Correct Answer: C. Erythema multiforme**