In cases like this, it helps to consider the two separate sentences for the singular and plural subjects:
- One child is dyslexic.
- Ten children are dyslexic.
Because the first sentence is true, we know to use the singular is.
For academic writing, if you're concerned about following conventional "rules" of grammar, it never hurts to consult the prescriptivistic grammar guide Elements of Style. Chapter 5 gives an example of a case in which a similar sentence would use the plural. The sentence is: One of the ablest men that have/has attacked this problem. If we use the same method of splitting it into two sentences with two subjects, we get
- One man has attacked this problem.
- Men have attacked this problem.
Because the second sentence is the true one, we know to use the plural have.
That said, in everyday speech, using either the singular or the plural should convey your meaning just fine. There's no issue of semantic confusion.