As I mentioned on Sheila's page, this would have been a different series without Bobby. Having all the kids about the same age would have ensured stability. Bobby, being much younger, was the wild card.
For one thing, he was the most aggressive of the group. It was perhaps inevitable that he would take the role of Barbarian; most of the other roles wouldn't fit him. As a wizard, he'd be as immature and careless as the Sorcerer's Apprentice. Bobby as a Sage or Acolyte? Forget it. He needed a role where he could bust loose; a role that no teenaged member of the group could embody without making people a little nervous.
For that reason he's given one of the more powerful weapons, yet one that is not invasive like Hank's longbow. The club is a blunt object of plain and simple power. It's an immature weapon for an immature user: throwing off great amounts of energy without really focusing it. Bobby seldom gets to use the club on a living thing (the rules of children's television being what they were) and confines himself to trees, mountains, glaciers. In "Servant of Evil" he uses the club to toss one of the lizardmen, but there's apparently no harm done.
He's not short for his age (Terry and Jimmy Whittaker provide the scale for someone Bobby's age) but he is the prepubescent odd man out in the group. Fortunately, he found Uni somewhere along the line and was able to displace the feelings of being the "baby" onto her. In the Tower of the Celestial Knights, Bobby's biggest anxiety is that he could BE a baby again. It not only renders him helpless and unable to defend himself, but reduces a group status he already thinks is a bit shaky.
Not that he's worried about Sheila. Their relationship is remarkably free of the sibling sniping sometimes evident in families. When Bobby disappears in "City at the Edge of Midnight", Sheila is frantic. When Sheila disappears in "Quest of the Skeleton Warrior", Bobby is frantic. He doesn't get so frantic, though, that he falls out of character. When Bobby and Sheila are reunited in "Servant of Evil", his reaction to Sheila's emotion is typical: "Quit the gushy stuff."
If anything, Bobby has his most obvious problems with Eric. Eric likes tweaking Bobby about his height and age, but this doesn't upset Bobby. He just responds in kind. Some of the byplay has a hostile edge to it (such as the opening exchanges in "Dungeon at the Heart of Dawn"), but it's not all hostile. By the time of "Cave of the Fairy Dragons" Bobby is able to take Eric's short jokes in stride.
Uni isn't just part of the group; she's treated as Bobby's exclusive pet. All of them seem to accept this, even Uni. Watch for a really nice moment in "Quest of the Skeleton Warrior"; with Sheila gone, Bobby searches dejectedly. Uni nudges Bobby, asking him to rub her head; you can tell she's doing it to try to console him. It's a nice defining gesture, one of those added details that made D&D unusual among the Saturday offerings.
About a dozen times in the series, the gang is on the point of going home, and the decision is made to leave Uni behind, repeating almost exactly the same words to a saddened Bobby each time: "She doesn't belong in our world." (It would seem that nobody in the group knew enough about medieval European history and culture to know that, once upon a time, unicorns DID belong in our world.)
Every one of the group has their own speech patterns, but Bobby's is almost too idiomatic. Phrases like "Gnarly" and "no way, Jose" tend to date both Bobby and the series. To me, however, fantasy always needs to be grounded at least a bit in our reality. Bobby makes the whole thing effective, but does so effortlessly.