It is narrated from Ibn Abi Mulaykah that Ibn Abbas (may Allah be pleased with him) said regarding the testimony of children: "It is not permissible." Ibn Jurayj narrated from Ibn Abi Mulaykah, from Ibn Abbas (may Allah be pleased with him), with these additional words: "Because Allah the Exalted says: 'Those whom you approve of as witnesses.' (__QUR_N__ al-Baqarah: 282)"
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1708
Hadith Takhrijاسناده صحيح اخرجه البيهقى 10 /162 - وفى المعرفة السنن والآثار له (5898)۔