Hi Kathy,
new student here, just reading through some old posts. For me, an essay and a sermon are two completely separate things, and the goal of each is completely separate. Therefore it is not just style which should be different (after all, who wants to hear an essay read out every Sunday?) but also content. Even a short essay would need to consider many more points than can be taken in during a sermon.
There are clearly many resources to do with preaching, but the one book that I always recommend is "communicating for a change" by Andy Stanley and Lane Jones. This really opened my eyes to a new understanding of what the purpose of preaching is and how that then influences how you go about preparing a sermon. What you do for the children's talk is pretty much the model for adults too! The book also addresses your concern about "freezing".
Hope this helps.
Regards,
Harvey