For this issue, I personally believe that children may lack a sense of security at multiple stages, depending on factors such as family environment, children's personality, and life experience. Here are some of the stages where children may lack the most sense of security:

1. Infancy: Newborn babies who have not fully adapted to the new environment and left their familiar uterus may feel unsafe and helpless. At this point, the companionship and support of parents or other caregivers are crucial for building a child's sense of security.

2. Kindergarten and primary school stage: When children leave their families for the first time to enter kindergarten or primary school, they may face many new challenges, such as adapting to new environments, making new friends, learning new skills, etc. These changes may cause children to feel uneasy and anxious, and parents need to provide more support and attention to help them build confidence and adapt to the new environment.

3. Adolescence: Adolescence is another important stage in children's growth process, where they face more challenges and pressures, such as physical changes, interpersonal relationships, and academic performance. These changes may cause anxiety and unease in children, requiring more support and guidance from parents and teachers.

Finally, it should be noted that each child's situation may vary, and the stage and degree of insecurity may also vary. Therefore, parents and teachers need to closely monitor their children's performance and emotional needs, provide timely support and attention, and help them build confidence and adapt to the new environment. I hope this answer can be very helpful to you. Thank you!