Have you ever heard of Dolch words? These are a list of 220 words that Edward William Dolch collected in 1936. None of these words can be “sounded out” because they don't follow the basic rules of phonics.
Even if you've never heard of these before, you may be surprised to find that your older kids still need practice with these. I can't count the times during the day that I hear, “Mom, how do you spell...?” I would be willing to guess that 9 times out of 10 the word my child is attempting to spell is a Dolch word.
My husband's standard answer to, “How do you spell...?” is “d-i-c-t-i-o-n-a-r-y”. In other words, “Look it up.” My kids usually groan when they hear this answer, but it really is a great practice for them to get into. In the meantime, I will continue to have my kids use www.spellingcity.com.