Conditionally load child themes

WordPress use an option to store the current active child theme. This option is named 'stylesheet'.

Using the filter 'pre_option_{$option_name}' you can change every option getted by WP.

So, in your case you should hook the 'pre_option_stylesheet' filter, use some logic to retrieve current season and return the child theme you want.
This workflow should be done in a plugin (loaded before theme and child theme), in this way you are sure it works.

Example plugin code:

<?php 
/**
 * Plugin Name: Seasoned Child Theme
 * Plugin URI: http://wordpress.stackexchange.com/questions/114538/conditionally-load-child-themes
 * Author: G.M.
 * Author URI: http://wordpress.stackexchange.com/users/35541/
 */

add_action('pre_option_stylesheet', function() {    
  $seasons_themes = array(
    // this are the child theme names (the folder name) related to season
    'child_winter', 'child_spring', 'child_summer', 'child_autumn'
  );
  $m = intval( date('n') );
  if ( $m < 3 || $m == 12 ) {
    $theme = $seasons_themes[0];
  } elseif ( $m > 2 && $m < 6) {
    $theme = $seasons_themes[1];
  } elseif ( $m > 5 && $m < 9) {
    $theme = $seasons_themes[2];
  } else {
    $theme = $seasons_themes[3];
  }
  $root = get_theme_roots(); 
  $path = false;
  if ( is_string($root) ) {
      $path =  WP_CONTENT_DIR . "https://wordpress.stackexchange.com/" . $root . "https://wordpress.stackexchange.com/" . $theme ;
  } elseif ( is_array($root) && isset($root[$theme]) ) {
      $path =  WP_CONTENT_DIR . "https://wordpress.stackexchange.com/" . $root[$theme] . "https://wordpress.stackexchange.com/" . $theme;
  }
  // if the theme exist return its name, if not return false
  // (theme setted in backend will be used)
  if ( $path && file_exists($path) ) return $theme;
  return false;  
});

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