You have to let them try things, even if you don't think it's going to work.  That trial and error process is what is so important, and when they see how something plays out, it may lead them to an entirely new idea that you would have never thought of if you had stopped them from doing something.

As for the logo, personally, I wouldn't do it for them.  I think that would take away a sense of ownership and pride, and it can be really fun.  It doesn't have to be complicated or involved.  If they don't have a lot of time, or if they can't think of anything, they can just use their team name in a certain font.  We used the "keep-scratch" method years ago picking a name, and all the kids could stand behind the final chosen name.

I think of myself as a facilitator.  That helps me to know what I should be doing and what the kids should be doing.  I can ask questions, help them set things up, etc, but they are the driving force and they are in charge of what direction to go.  As my kids have gotten older, they do much more on their own now, too.  I can give them things to do on their own time and then they report back to the group, for example.

Most of all, have fun with them!

Sonya