Life for children with written output struggles is intensely frustrating. You will usually hear comments such as "everyone's faster than me, I am always finished last, I wish I was like the other kids," etc... now the fact that your ma or da is telling you you're great, grand, dandy, smart, talented doesn't always cut it.
However, pointing out that Winston Churchill was uneven at school can help momentarily stem the torrent of despair. May be most affective if the child has a regard for Churchill however, could always create a nice pause when they reply... er, who? For the record according to the book we just read Mr Churchill was sometimes the first in his class and sometimes last.
I watched a video about learning differences which featured all kinds of people including the formidable artist Robert Rauschenberg (sp?), whose description of his mother or parents saying "we just thought you were stupid..." truly sent me crumpling into the couch. God bless him for finding the strength to lift his paintbrush in-spite of the prevailing winds.