I am lucky, I have two den chiefs for my Webelos den. Both just recently completed den chief training (February) and in June received their Star rank. They are 11 and 12 years old, just completing sixth grade (about two years older than the Cubs). Being a ASM and a Webelos Den leader I have access both Troops, Dens and Packs. With two den chiefs, I was initially going to have each boy come every other week to the den meetings. What I found out is that they like each other's company and would rather come every week and work together! If you can get two boys, this is great! Sometimes they organize games and one leads team 1, the other team 2.
Another bit of advice, never have a den chief try to lead his younger sibling!! Another advantage of having two den chiefs.