Individuals who had new children during the coronavirus disease 2019 (COVID-19) pandemic became parents in challenging situations, starting from pregnancy and continuing to after birth. This study aimed to clarify the characteristics of parental loneliness, perceptions of parenting, and psychosocial factors among parents having new children during the COVID-19 pandemic. The participants comprised a first-child group (523 parents; those who had their first child) and a second-child group (621 parents; those who had their second or subsequent child). We used webbased questionnaires to explore parental loneliness, perceptions of parenting, and psychosocial factors (distress, parental burnout, well-being, marital satisfaction, and social isolation). Participants answered the questionnaires in November 2022, during the eighth COVID-19 wave in Japan. We compared the groups and subgroups according to parental sex and determined the relationship between variables. The parents in the first-child group felt lonelier than the parents in the second-child group (p < 0.05), and their loneliness was correlated with psychosocial factors. Significantly, more mothers in the second-child group answered "agree" to negative perceptions of parenting than mothers in the first-child group. Additionally, parenting difficulties were correlated with a negative perception of parenting and parental burnout in both groups. Furthermore, providing parental support may improve parenting and promote parents' health.