The child was airlifted to the John Radcliffe Hospital in Oxford after a serious crash involving a red Ford Focus carrying four people heading towards Windsor and a silver Citroen Xsara heading in the other direction.
The collision happened near the BP petrol station and the junction to Fifield Road at about 6.30pm.
Police said they were continuing their investigations into the crash but no arrests have been made.
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The Met Office has issued a yellow alert for Windsor and Maidenhead, Slough and Wokingham, warning that rain moving eastwards across the south of the country may turn to snow in some places.