"Uncaught TypeError: undefined is not a function " jQuery each function issue

visibleDivs is a number :

var visibleDivs = $(this).children('div').length;

So it doesn't have a width function which make this line fail :

visibleDivs.width(targetWidth);

You probably want

$('.colourbox').each(function(i){
   // select visible children
   var visibleDivs = $(this).children('div');

   // use whatever width calculation you'd like...
   var targetWidth = 300 / visibleDivs.length - 1;

   // apply the width to the divs
   visibleDivs.width(targetWidth);
});

But you should have solved this issue yourself using the developer tools of your browser :

1. look at the exact line of the error given in the console
2. if that's not enough, debug and, line after line, look at the value of your variables

Variable visibleDivs has a value of a number of child divs.

var visibleDivs = $(this).children('div').length;

What you probably wan't to have there is a list of child divs instead:

var visibleDivs = $(this).children('div');

Full example:

$('.colourbox').each(function(i){
    // select visible children
    var visibleDivs = $(this).children('div');

    // use whatever width calculation you'd like...
    var targetWidth = 300 / visibleDivs.length - 1;

    // apply the width to the divs
    visibleDivs.width(targetWidth);
});