Why Kids Resist Reading Music
Most kids resist reading music like a trip to the dentist. And the younger they are, the more they resist. Another area of resistance, playing with both hands, is entirely related. Brain hemisphere coordination is the cause of all these difficulties. As background, the left brain controls the right hand, and the right brain controls the left hand.
Additionally, there is a "connecting highway" of neurons between the two sides of the brain called the corpus callosum. This crucial neural connection is more tenuous and even non-existent with younger children.
Brain Hemisphere Coordination
Let's start with reading music. It requires so many dimensions that a child of six is usually completely confused. Your requests to use all the dimensions at one time (up/down, left/right, white/black, slow/fast,etc.,) completely overload them. So with some kids, not just the youngest, you are asking for the near-impossible when you ask them to read music.
This is because their brains are not developed to the point where they can handle this much information, bouncing from one hemisphere to the other. In contrast, older kids, and some younger ones, grasp the complexities of notation right away. This is because their brains and the connections are much more developed. Try teaching reading music to an adult and you will have much more success since their brains have finished developing.
Kids Find Music Reading Tiring
So experience tells us that kids respond to reading music when they are allowed to decide when enough is enough (when their eyes roll back in their head.) Actually, it's a question of dosage. The average piano teacher has a dosage of 100% reading music. I prefer 20% until kids show they can take more.
I also examine kids to see if they have the "precursor" skills needed to read music. For example, without the firm, instantaneous ability to distinguish left from right, and up from down, reading music fluently will be all but impossible.
And for the youngest, even left and right is confusing. At the piano, "up" is to your right, and "down" is to your left. Try that on a five year old. Left and right are confusing to younger kids. Lower the bar, the younger the child. Show them easy victory and build from there.