Dear Sir/Ma,
I write with humility to seek for the job of a babysitter in your home. I have zero professional certification in this job but, I can assure that your child(ren) are in safe hands and you would not have any regret whatsoever.
Growing up, I lived in a family compound where parents sometimes left their children during the holidays to go to their business places so, as part of the order cousins, I had the job of keeping them busy and playing with them.
I promise to face my job squarely and not give you reasons to worry.
I look forward to your reply.