ifChild = function ifChild(el, cssPrefix, suffix, fn) {
  var c = el.querySelector('.' + cssPrefix + '-' + suffix);
  c && fn(c);
}
Function checking if a specific css class exists as child of element.

@param {HTMLElement} el element to find child in
@param {string} cssPrefix css prefix of button
@param {string} suffix class name
@param {function} fn function to call if child is found
@private