All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
If a person touches baby's private parts, while changing diapers,with the palm of his hand then his ablution becomes invalid. This is because it was narrated from BusrahbintSafwan that the Prophet (PBUH) said:
"If any one of you touches his private part with his hand, let him perform Wudu'." [An-Nissa'ee].
However, if the afore person didn't touch the baby`s private parts directly with his hand while changing diapers or cleaning .i.e. by using tissues, gloves or with the back of his hand, then his ablution remains valid. And Allah knows best.