Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 38
Question: If a woman touches the ‘awrah of an infant, does this invalidate her ablution?
Response: This does not invalidate the ablution of the woman – if she washed her child and touched his private parts. Even if a man touched his private parts, this does not invalidate his ablution unless he did so with desire. If however he did so without desire, then [renewing the] ablution is recommended but not obligatory.