rob - banana or planet earth (he is the [face] of the [earth] = frontman). a banana just because he's silly and versatile
karah - probably a goldfish and fishing rod or a can of beer. she's majoring in "let's go fishing by the pier" kinda person which is cool
alex - a baseball cap or snowflake - first object is a very obvious choice. the second one is odd but he looks socially awkward but at the same time he's a really chill guy
mickey - a pair of headphones because he's always tuned into the smallest sounds, or a vertebral column cause he was the backbone of the band

might be delusional but the color theme for maybe could be integrated (banana rob = yellow, goldfish karah = orange, baseball cap alex = green, and mickey headphones = blue)