Napaak Child On Mother During Salaah

If a child (with impurity) sat on the mother on its own, then the mother’s Salaah will be valid as the impurity will not be attributed to the mother. However, if the child does not have the ability of holding (onto the mother by itself) and it remains on the mother less than the duration it takes to discharge a Rukn, then Salaah will be valid, but if it remains on the mother for the duration of a Rukn or more, then the mother’s Salaah will be invalid.

Coming to your query, if the child was placed on the back by the mother then her Salaah will be invalid irrespective of whether the child was Napaak from the beginning or the child messed whilst the mother was in Salaah with the proviso that the Napaak child remained on the mother for the duration of a Rukn.