When a child has a secure attachment to their caregivers, the child feels content when their caregiver is around. The child also becomes upset when the caregiver gets separated.

When a child has a secure attachment, long-term effects include acceptable emotional expression, peer relations, and social skills. A child who has secure attachment may have a greater understanding of other individuals emotions. Those children who have a secure attachment are less aggressive and have less antisocial behavior as well. (Sincero, (n.d.).
The relationship a child has with their caregiver is a prototype for future relationships the child will have. The child establishes trust in the world, and knows that they will be cared for, and allows for emotional regulation as well.
The more secure a child is with their caregiver, it will be easier for the child to become an independent adult. A child that is secure will be confident and resilient, especially when they are confronted with peer pressure at school or social events.
Reference:
Sincero. (n.d.). Bowlby Attachment Theory - Core emotional development in humans. Retrieved June 06, 2017, from https://explorable.com/bowlby-attachment-theory

How Insecure Attachment Impacts Development

There are different kinds of insecure attachments which include ambivalent attachment, avoidant attachment, and disorganized attachment.
Ambivalent attachment is when a child becomes upset when they get separated from his parent, which can mean the child doesn’t feel as if he or she can’t rely on the caregiver when the need arises.
Avoidant attachment stays away from his parents, those children to show avoidant attachment may have parents who are neglecting or abusing them. Disorganized attachment is when there isn’t a clear attachment between the child and the caregiver (Sincero, n.d.).

Reference:
Sincero. (n.d.). Bowlby Attachment Theory - Core emotional development in humans. Retrieved June 06, 2017, from https://explorable.com/bowlby-attachment-theory