nine-year-old william kulk died in hospital on sunday night .
his eight-year-old sister piper died on sunday morning .
they were both involved in a horrific car crash on the nsw central coast .
over $ 32,000 has been raised to help the devastated family .
were travelling with elder brother , mother and grandmother .
they had spent the day at the royal easter show in sydney .
their brother daniel , 12 , is in a stable condition in westmead hospital .