smee - I forgot to say
if you child has visual discrimination issues the best thing to do are word searches, where's wally and spot the difference.
Not exactly sure how you'd know your child had visual discrimination problems, but I think it would show up as a) hating the above puzzles, and b) spelling problems. They wouldn't be able to look at a word and see if it was spelled right or not.
All of my 3 DC have convergence and eye tracking issues. DD has been misdiagnosed with dyslexia. Her eye tracking problems means she just can't read at all. We've done a course of manual vision therapy, and it wasn't enough, and now we're starting computer vision therapy.
DS1 and DS2 are both perceived (by school) to be excellent readers. Top of the class type stuff. Yet DS2 hates reading - it turns out he has really bad convergence issues, and can only focus at one distance. And DS1 skips words, skip lines, spells badly, and can't read phonetically - he has convergence and eye tracking issues. But he reads for hours, and loves reading.
So DS1 and DS2 will also start computer vision therapy soon. :)
So, my conclusions are that it is a reasonably common problem, and normally goes undiagnosed and untreated.