the Messenger of Allah said, about urine of a male child that suckles: "The urine of the boy is sprinkled, and the girl's urine is washed." Qatadah (one of the narrators) said: "This is so, as long as they do not eat, when they eat, then both of them are washed."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ فِي بَوْلِ الْغُلاَمِ الرَّضِيعِ يُنْضَحُ بَوْلُ الْغُلاَمِ وَيُغْسَلُ بَوْلُ الْجَارِيَةِ قَالَ قَتَادَةُ وَهَذَا مَا لَمْ يَطْعَمَا فَإِذَا طَعِمَا غُسِلاَ جَمِيعًا