If marhooma didn't have much wealth in inheritence and with all her money the qaza fasts and namaz couldn't be completed. Then is it necessary for her child to pay from their pocket and get the remaining qaza namaz completed? What if only child is a daughter and doesn't earn money and is living on her son's and husband's income plus pocket money?

If the daughter is unable to pay, there is no obligation. She should try herself to do the qadha. If not maybe she can get help from her marjie from khums to pay someone to do it as she is poor