You might get some good ideas and words to use from the article on the following site. ScienceDaily: Screening Children For Heart Disease Risk Helps To Identify Parents At Risk
I would suggest starting the letter with just an introduction of who you are and the service that you will be offering and providing. Also, that you want to emphasize preventive steps that can be taken against heart diseases. Mention that this is a follow-up letter to information that they have already received? (I don't know what information has already been given to the target audience, nor in what format it was given.)
A letter could be worded to attract the attention of parents who already know that they have medical conditions which can be carried on to their children.
For instance: "If you have any of the following conditions, it is possible that you children may also be susceptible to..."
Of course you don't start the letter with something like the above sentence above, but put it more towards the middle of the letter.
I wouldn't make the letter too long. Short and to the point. Make it like an invitation for them to come and see what you have to offer. Your letter is basically an ad, so it needs to be an easy read and thus easy to remember. If there is any competition for what you are offering, I would suggest trying to be the first to offer your services.
You could also put something in the letter about "If you have any questions, then please feel free to give me a call at..."
This is just a possible start. Hopefully, others will come along and add their own ideas. But, in the end, the letter must be written by you because it will represent yourself and what you do.