父母延期explanation letter請教

父母三月來美幫我們照顧剛生的孩子，現在想申請延期。正在考慮如何寫申請信，什麽樣的理由容易獲批準？
１。遊了東岸， 西岸還沒看。
２。孩子(5 months old)有medical condition that requires special treatment. We need grandparents to take care of our son so both of us can work to afford the expensive cost that is not covered by insurance. (Sadly, this is ture).

能否提供樣本？sample letter.

最近延期形式怎樣？

小弟多謝了！