Different room designs often dictate your ability to control a crowd solely. I always ask for assistants though, it is much easier to watch a few assistants in stead of a few hundred people running around. I'll try here to break up this topic into three sections. 1) small rooms - speaking only, 2) medium room - prop stations set-up, 3) Large Room.

Small rooms 

With no props a small room is for the brave of heart. I like this format with very young children, although it comes off much better with just a few photos to pass around and a few small props. Your job is to be the speaker, no matter which room type you have. Assistants rarely have any knowledge of the equipment they are showing and protecting. So nearly all the knowledge in the room is in your head.

Sit facing the door with the crowd in a circle or normal room set-up. Facing the door is so you can easily keep track of new people as they enter, without you needed to always turn around. Small kids expect a sit around. I like it too. Get down on the floor with them and enjoy their bee sting stories. That's really what your lecturing to children is, a fun time. If you get some science through to them, that's great. But be happy if they are entertained by you and your props. 

Older crowds will most likely sit directly forward facing you. Passing props around is an easy way to keep these people awake and it allows you to pace yourself between topics. I use assistants in smaller room to help with passing out flyers to those coming in and leaving. They are there to communicate with others OUTSIDE of the room. If you need to be interrupted, then a assistant should be the one doing it. 

Medium Room - props

Most of the time you will lecture under this category. A Midsize room usually has areas that you can not see easily from your central lecture point. These areas are idea for your set-ups and props. I find that most often a rectangular room can offer 2 display areas which are well away from your lecture area. It is important to keep your hands-on props in an area where people can feel free to talk about the items. So keeping stations off by themselves allows you to speak to those who wish to hear you, meanwhile your props and assistants are busy off handling a big chunk of your crowd.