I have seen in many movies that ghosts are always visible to kids eyes. Is there any supernatural reasoning behind this?
Why are ghosts commonly visible to kids eyes?
I think you'll find that there are billions of different explanations for this one. It is very common to the majority of cultures to say that kids are more sensitive to the supernatural and are more capable of seeing ghosts.
One of my favorite explanations is that kids are new to this dimension, so they still have memories of the spiritual plain and do not know that isn't normal to see ghosts. This means that they would grow up and adults will teach them what is normal and what is not.
However, the most "scientific" and common theories are well explained here:
1. Kids Brains Are Proportionately Larger Than Adult Brains
[...] a toddler would see no difference between a TV character and that soul of his or her deceased grandmother. This makes it easy for them to establish conversation and relationships with them.
This somehow connects with the more supernatural idea of the dimensions where adults need to show a child what is real or not. However it is more scientific than that.
2. The Brainwave Connection
Their brains see and hear things that only they understand. They have the power to talk to their imaginary friends, enjoy the world through unfiltered eyes, and not be forced to believe one concept over another.
3. Between the Ages of 11 and 14 Parts of the Mind Shut Down
Unless a human being makes every efforts to hold on to the neural connections that are psychic or that can see spirits/ghosts, they will be completely erased. The brain stops “seeing.”
I think that you'll also find that not only children, but animals (mainly dogs, in movies), are always very commonly able to see ghosts. They are also influenced by the similar reasons as kids, but with some slight differences. Animals are, such as kids, not held by the reality molded by adults, and are very perceptive of things that adults easily ignore. Their heightened senses allow them to see very minimal changes that we are incapable to see, that are culturally interpreted as "seeing ghosts".
Comparatively the child brain is 135% larger than the adults. The child is also not framed by an adults conception of reality and can, rather, make their own decisions. When children turn 11-14 parts of the brain actually shut down so psychologically their could be something in those brain synapses that affect it. I also think for movies it is more believable because children are more likely to see "something" and believe it to be a ghost.