24 inch should be plenty for a 2 player 6 button a real streetfighter 2 panel is 26.5 inches wide and you are doing a smaller machine. My control panel is pretty comfortable at 21 inches. As for themes, if it is primarily for the kids see what games they like and go for some kind of Vs machine, If you have a Boy and a Girl you could have Pink vs Blue, Kirby, Pinky, Princess Peach Vs Sonic, Bomberman, Bob (Bubble Bobble). If you have 2 Boys there is always the classic Marvel vs Capcom or DC Vs Marvel superhero/fighter mashup.
I just went with a fairly simple pacman maze background and Pacman himself big and centre. The control panel is a very simple take on the original control panel.