An essential principle of good teaching is not to impose activities that children find taxing. The fallout from not adhering to that principle is enormous–both physically (in terms of causing actual strain) and psychologically (in terms of sapping the child’s motivation).
At the same time, it’s important to keep in mind the fact that young children can often achieve far more than is commonly assumed. We see evidence for this all the time. For example, young children who are raised in bilingual homes easily master two languages. Further, the “double” learning actually ends up enhancing their overall language development. Yet the same age children would not do well if the second language were taught in a classroom through traditional techniques. The key is offering the right environment and the right materials that enable the skills to flourish.
The analogy to a second language is useful because in many respects, reading is like a second language. It is the language of speaking transformed into the language of print. If the teaching is carried out in a manner that is appealing to the children and well-suited to their skills, the benefits are considerable. And just as the early learning of a foreign language proves to be beneficial, so too does the early learning of reading.
The basic requirement is that your child should, without strain, be able to work on activities for 15 to 20 minutes at a time