Looks fine - some minor changes might help:
One day, a boy and his sister, from a rich family, were walking in the park when they saw a thin and frail elderly man.
They decided to help him by giving him some money, so they put twenty dollars in one of his shoes.
They also wanted to find out how the man would react [what the man would do] when he saw the money.
Ivy said, “Mum, he looked so weak and poor.