each other's (hand/hands)/(child/children)

!. They held each other’s hand.
2. They held each other’s hands.
3. Sita and Jaya keep each other’s child.
4. Sita and Jaya keep each other’s children.
Which of the above are correct?

All are grammatically correct but I’d rather choose 1 and 4. We normally hold just one hand to express compromise. And nowadays most families have 2 or more children. However, indeed, that depends on you.

A native English speaker would not use ‘keep’ there. They would use ‘look after’.

Sita and Jaya look after each other’s children.