the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
If she is in nappies, then there is no harm if you change her diaper in the presence of her mahram family members unless you fear that some trials will occur because of it or you fear that someone will be offended.
This is a matter of practical necessity. However, her private area should remain covered at all times when there is no need to expose it.
Source: Islam Today