What is the ruling on the prayer of one whose daughter climbed onto his back during prostration, preventing him from rising for the next unit of prayer, so he prayed it sitting, whether he was praying alone or as an Imam, and what should the congregant do in this situation?

If a child sits on the back of a worshipper during prostration, he is not permitted to pray the rak'ah sitting, because standing is a pillar in obligatory prayers for one who is able. The Prophet (peace be upon him) used to gently lift al-Hasan and al-Husayn from his back without interrupting his standing.

If the Imam prays sitting intentionally, and the followers intentionally follow him, then their prayer is invalid. However, if this was due to forgetfulness or ignorance, then their prayer is valid according to the Hanbalis.