Complex sentence: Initially the Jewish refugee agencies saw 5,000 as a realistic target but, after the British Colonial Office turned down the Jewish Agency's request to admit 10,000 children to Palestine, the agencies increased their request to match this number.
Split into simpler sentences: Initially the Jewish refugee agencies saw 5,000 as a realistic target. However, after the British Colonial Office turned down the Jewish agencies' request to admit 10,000 children to Palestine, the Jewish Agenciess increased their initial plans (and request) to match this number.