Why do kids call their parents by their first name?

When I've seen this it's because there is a lack of connection or affection within the relationship.. Also to be blunt I think it is also a sign of disrespect.

I have personally seen so many kids call their parents by their first name. It definitely isn't a sign of disrespect. Your deeds show your respect to your elders. Calling them mom or dad isn't necessarily a sign of respect, It's just something you've learnt to do, something that has been passed on to you. It would be totally unfair to point fingers at someone whose traditions and upbringing differ from yours.

Because they don't feel close enough with them to address them by mom/dad. This is usually an attempt to distance oneself from parents, and not wanting to associate yourself with them as family.

Many reasons the positive ones is because they have a relationship based on friendship with their parents

Kids call their parents by their first name because they discovered that their real name isn't mommy or daddy and it became more interesting

I did this for a while with my parents, to be honest I think I did it because I didn't like them at that certain time. I didn't want to show them motherly or fatherly love for while.

First, not all children do. Second, it's likely societal influence on children. In a more liberal country, children have the liberty to call their parents by their first name. However, in more conservative cultures, proper honourifics and respect must be given to their seniors. In this case, their parents. Cultures start off as habits— Habits that become commonplace and accepted as a norm, and as a norm, becomes a culture that is slowly passed down. Society can also have said impact on the individual's actions and line of cognitive process. Their beliefs, their habits are all ingrained as such.

Usually kids call their parents by their first names out of respect, or because they were raised in such a manner. But some children do it out of spite and like to use a sarcastic tone to antagonize their parents.

it could be the child has heard it from others and it's then up to the parents to correct their child and tell them "that's what grown ups call us but you must call us mummy/daddy".