This is an interesting question. I had to do a little research to answer it. "Dare" can be used as a modal verb, but it can also be used as an ordinary verb.
As a modal verb it does not conjugate or have tenses. This is the same as words like "would", "could", "should". They have no past tense.
In your example, "dare" is used as an ordinary verb, so it does have a past tense. Your example is not modal. If you wanted to use "dare" in a modal way you would have said something like this:
"When I was 7, I knew that I dare not sleep alone".