Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Parent] and [Parent],
I am so sorry to hear about [Child]'s illness. It is unfair that this would happen to any child, especially one so [adjective] and [adjective] as [Child]. Everyone [loves/adores] [Child] and you should know that we are all praying for [him/her].
Your worry and pain must be overwhelming, but I know your love for your family is just as powerful. It will help you through this difficult time. Your friends and family will also help you. I am always willing to lend a hand. I could [specific task you are willing to do]. I could also [another specific task]. Also, I am always available if you just need a shoulder to lean on or somewhere to vent.
I will check with you soon to see how I might help.