Praise be to Allah.
Wudoo’ is not invalidated by cleaning a child’s urine or washing his diaper, because touching najaasah does not invalidate wudoo’, but it is essential to wash off the najaasah if one wants to pray.
It says in Fataawa al-Lajnah al-Daa’imah:
Wudoo’ is not invalidated by washing najaasah from the body of the person who has wudoo’ or elsewhere. End quote.
Majallat al-Buhooth al-Islamiyyah (22/62).
In Fataawa al-Shaykh Ibn Baaz it says (10/139):
Touching medical equipment or the floor of the washroom barefoot does not invalidate wudoo’, but if there is any najaasah on the floor and a woman or man steps in it, this does not invalidate wudoo’, but they must wash their feet if they stepped in it when it was wet, or if there is any wetness on the foot.
Touching a child’s clothes that are wet with urine does not invalidate wudoo’, but the one who touched it when it was wet has to wash his hand. The same applies if it was dry but his hand was wet – he has to wash his hand. End quote.
Shaykh Ibn Baaz also said: (10/141):
As for touching blood, urine and other impure substances, that does not invalidate wudoo’, but he must wash off whatever gets onto him. End quote.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about a woman who washed her child when she is in a state of purity. Does she have to do wudoo’?
If a woman washes her child, boy or girl, and touches the child’s private part, she does not have to do wudoo’, rather she only has to wash her hand, because touching the private part without desire does not necessitate wudoo’. It is known that when a woman washes her children, desire does not even enter her mind, so if she washes her child she only needs to wash her hands to get rid of any najaasah that has got onto them, and she does not have to do wudoo’. End quote.
Majmoo’Fataawa Ibn ‘Uthaymeen (11/203).
And Allaah knows best.