One of the biggest influences for me, as far as design process, comes from material published by 1114 (simbotics). If you aren't aware, these guys are known for producing robots that dominate year after year.
I suggest the Strategy and Robot Design powerpoints.
http://www.simbotics.org/resources/workshops
As far as getting kids involved, remind them that this is their robot. I still don't know how to get kids interested if they aren't, but those who are, just tell them to speak up. No one bites (hard, atleast).
To avoid any emotional-attachment issues, make sure its clearly stated that "We aren't using your design because we don't like you: we aren't using your design because its not whats best for the team". Remind everyone that "we are working for what's best for the team, not what's best for our egos."