If you will be speaking to multiple audiences, perhaps more strategy and preparation will be necessary. It is possible to speak to multiple audiences at the same time and it can be successfully, if done carefully.
I have gone through this scenario multiple times and thought it came out fabulous. The key is to discuss information that is suitable to all of the multiple audiences. For example, animals pretty much appeal to everyone, so what I had to do was vary the complexity of the animal facts so that everyone learned something each audience was satisfied. I would talk for a minute on a preschool age level, then elaborate on that for the older kids in the audience. The kids and adults stayed interested and involved as I progressed in complexity.
I have seen other presentations where multiple professions were present at the same time. Targeting all of the different professions can be as tough as appealing to various ages. The key once again is to find a topic that is universal. Speak and then vary your words to target each of those professions. Before any of this can happen, you must know what professions are in the room– having the audience raise their hands is a great and easy way of going about this and it adds a bit of interactivity. If you have majority of engineers in the room, perhaps the choice will be to tailor most of the presentation to the majority of the audience and then double back to accommodate the others. If you have a good variety then the best solution would be to keep all interested by randomly targeting each of the audiences.