When you refer to EFC, is that the FAFSA EFC, or an EFC as calculated by the college that your first kid will be attending?
If the first kid's college has a reliable Net Price Calculator, you could use that with different inputs to check out the various scenarios and get a reasonably good idea as to whether or not spending down the 529 faster will be more beneficial at that college as opposed to evenly distributing the 529 among your kids. Since every college has their own process for awarding need-based institutional aid, how this will impact your other kids at different colleges is anyone's best guess.