The answer of the eighteenth riddle - Striking Resemblance
The first paragraph.
Two child which strongly resembled each other
One was fine, another was missing
So the curious children came to investigate.
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» There're two person mentioned to have an very much alike appearance, both a child. One of the child was missing, so the other children (who may or may not the missing child's friends) came to...er...investigate.
The second paragraph.
They found the child guardians weren't presented
Were far away for a while.
They asked and knew where the child had gone to.
And realized there's an accident happened on recent
That the child might be the one was hit into
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» The children (which could only referred to Detective Boys...of course!) found out that the child's parent was away to somewhere and where the child would've gone to. They also heard about an accident which happened there recently, and the child might or might not be the victim of the said accident.
The third paragraph.
They finally found her in a familiar good care
A kind good-looking doctor
At where the child was resting in
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» They finally found the child (which was now 'her') in a hospital or a clinic of the doctor they knew or/and have met before.
The Conclusion
The riddle ask for the place the child was found at, which was Araide-sensei's clinic in the episode 427-428. The child's name is Amemiya Shoko-chan, the one which strongly resembles Haibara in appearance. She wasn't hit by a car, just lost consciousness because of the shock and the lack of sleep.