It's considered that children without siblings grow up spoiled and selfish. On the one side, this is true. They can be selfish because they are used to being treated as the only one for their parents. The result is that they think only about themselves. Also children without siblings are not ready to be responsible and face problems independently, while children who have siblings of their own are thought to care about their brothers & sisters. But on the other side, I think it depends on parents, namely, the way parents bring up their children. If parents teach their children to be responsible and give them the opportunity to be independent and not trying to indulge all their desires. Therefore, such children will not be spoiled even if s/he is an only child.

It's considered that children without brothers or sisters grow up spoiled and selfish. On the one side it's right. They can be selfish because they are used to being treated as the only one for their parents. The result is they think only about themselves. Also children without brothers or sisters are not ready to be responsible and face problems independently, while children who have brothers and sisters are thought to care about their brothers and sisters. But on the other side I think it depends on parents, namely, the way parents bring up their children. If parents teach their children to be responsible and give him an opportunity to be independent and don't indulge all their desires, such children will be not spoiled even if he or she is an only child.

I eliminated only children because this expression is a contradiction (only is singular, children plural). You would have to use 'only child' only for one person.