They need to learn to take tests
However, every job gives you a test, you need to take tests in college, you have to take a driving test, how do you prepare your child for test taking if you are not giving them the "formal education" that everyone says children need?
One of the harder parts of getting unschooling is seeing how the world is contained in the world, so to speak ;-) When things get pulled out and treated as artificial as they are in school, we get the feeling they are artificial. But they aren't. It's only the artificial reproduction of them in schools that doesn't exist in real life.
Before you asked for ideas on helping a child learn the sounds of the alphabet. But if you look at your older kids, was grasping that so difficult for them? The sounds are there in real life. If they weren't, then why would we learn them (formally or informally). Walking is useful so kids walk. Talking is useful so kids talk. The sounds the letters make are useful for figuring out what the on-screen commands in a video game mean (among loads of other things!) so kids figure them out. ;-)
As people said, tests are a part of life. Our mouths will pass or fail us on how well we've read directions or a recipe. ;-) Tests are also part of real games people do just for fun. Video games, logic games, crossword puzzles, computer games, personality quizzes and so on.
I think the one lesson about tests that kids don't need is that their entire future rides on how well they do. That message is way too prominent in school and kids have tests thrown at them every which way. If your kids want to take the SATs and know there are books and software and classes to familiarize them with the style if they feel it's necessary and they know they can take it as many times as they want (as far as I know!), then it's not a big deal. (Peterson's and College Board (SAT) have free practice tests.)
BTW, there's a lot of information about unschooling and getting into college at Learn in Freedom.