Answering the question on Quora: "How do I specify the loading order of plugins?" by changing the question to "How do you ensure that you declare your child class after the plugin's class is declared?" - http://www.quora.com/WordPress-Plugin-Development/How-do-I-specify-the-loading-order-of-plugins. Note I've boiled this example down to its bare essentials so the reader can understand the technique without concern for the best plugin code architecture which is a much bigger question. If you'd like to learn more about plugin code architecture check out our blog at HardcoreWP.com.