"How can my child understand if he reads so fast?"
Here is how you teach your child to speed read...
When you start your child's speed reading exercises, you start by getting them to visually read the words as fast as possible.
At this stage, your child doesn't have to understand what he has read.
Keep on practising.
Once your child is used to the fast speed of reading, they will eventually understand what they read.
So first, learn how to speed read. Then the understanding will come eventually.
It is essential to practice as much as you can.
The right brain education library comes with lots of exercises for your child to practice their speed reading.
Take your time and practice consistently.