The labs are not intended to teach a subject but to reinforce what has already been taught.
Make sure the kids read the experiment completely before they start. This can be done for homework or together in the class. It can either be done the day before so the ideas have a little time to percolate So, read about the topic in a textbook beforehand.
Spend some time beforehand discussing the lab, procedure, hypotheses, possible outcomes, safety.
One teacher has a team demonstrate the experiment, or at least how to set it up, before the class does it. Give this job to a different team for each experiment. I prefer to do without the demonstrations because, although they make things go more smoothly, I think the kids get more out of the experiments if they have to figure things out for themselves.
Some teachers, especially in the youngest grades, prefer to have the materials distributed to each team. It's certainly faster this way.
If more than one class is going to use the kit, I recommend strongly having a parent or other adult be responsible for repacking and checking all of the equipment. An alternative would be to select one team for each experiment that is responsible for the equipment. This may teach responsibility and excourage ownership for the equipment. But the risk is that, the next time someone goes to use the kit, not everything is there.
I prefer to have the kids work in pairs. Some of the experiments require two or three kids because extra hands are needed. The rest may be done by a single child but they'll miss the very important discussions and idea exchanges that happen with a partner. With three or more kids working together, someone ends up with nothing to do.
Selecting partners for science is no different than for any other activity. I suggest giving careful thought to partners since it is important that each child is actively involved. You'll have to find out what works best for your class: anywhere from a carefully planned pairing to drawing names out of a hat. You might change partners each time to give the kids a wider range of ideas and experience, or keep the same partners all the way through a unit for improved efficiency. If this is your first experience, I suggest drawing names out of a hat each time until you see how things go.
One teacher likes to have her first grade class stay together: they read a step and then perform it, read the next step and then perform it . . . Certainly in the older grades, the teams should progress at their own pace.
If the program is successful, they kids will want to try other things. I require that they first finish the lab as written and then go on as long as their experiment is relavent and safe. I encourage them to document and share their discoveries with the class.
Some teachers appreciate having extra adults help oversee the actual experiments. One teacher would have one parent for each table of two teams if she could get the volunteers. Many teachers like having one extra adult to help keep an eye on things and clean up the inevitable messes. Some teachers prefer to go it alone. If the equipment is to be used by more than one teacher, I recommend at least having a parent to make sure the kit is kept in good order.
Since documenting your work is important to the scientific process, I encourage teachers to check the labs to be sure they are properly filled in. You're not looking for "right" answers, rather that hypotheses, observations and conclusions are clearly written.