1. Context of the study About 123 people migrate from Mainland China to Hong Kong to reunite with family every day. Migration from Mainland China to Hong Kong can be highly stressful. Newly arrived mothers and their children experience adaptation-related stressors, such as lifestyle changes, unhealthy housing conditions, and family financial restrictions, and they tend to experience high levels of psychological distress.

   Among the migrant population in Hong Kong, a particular group is known as cross-boundary children who engage in everyday-migration. Most cross-boundary children live in Shenzhen with their parents, who are not Hong Kong citizens. To attend school in Hong Kong, these children have to cross the border checkpoints connecting Hong Kong with mainland China every day. Families with cross-boundary children experience adaptation-related stressors, such as language difficulties, economic burden, and unclear citizenship status. These families have weaker social support networks in Shenzhen and seldom use community services in Hong Kong. Thus, cross-boundary families face multiple challenges and stressors due to their situation, which also puts them at a high risk of mental health problems.
2. Target of emotion regulation Newly arrived and cross-boundary families face similar problems, such as discrimination, acculturative stress, mental health problems (e.g., depression). All these problems can result in family conflict between parents and child. For example, early interviews have revealed that conflict over schoolwork and academic performance between children and parents could threaten overall family harmony. Frustrated with their lack of emotion regulation strategies, the children and parents often vented their anger and anxiety on each other, which sometimes led to physical abuse.

   Previous studies have demonstrated that the identification, comprehension, expression, regulation, and use of emotions are important predictors of individual adaptation and are positively correlated with mental health (e.g., happiness and social functioning). Parents' emotional regulation and mental health can influence their children's emotional socialization by means of diffusion through the family emotional climate. Thus, a community-based intervention focused on emotion regulation might help decrease these conflicts and enhance family harmony.
3. Cultural and contextual considerations in the intervention feature Family system theory states that families function as interdependent individual systems in which each member can impact others. It has been found that children's emotional problems are related to the stressful experiences and emotional symptoms of their parents. Therefore, we involved both parents and children concurrently in the intervention to maximize the intervention benefits. The parallel-group model is widely advocated in family-based intervention, but its implementation has a certain degree of difficulty. A parallel-group comprises two concurrent, but separate intervention groups (e.g., a child group together with a parent group); participants from the two groups engage in similar activities in different rooms with corresponding goals and contents. Compared with an intervention for individuals alone, involving family members in a family-based intervention can lead to greater benefits. Separating parents from children could prevent children from their parents' over-control and let each group deal with their own problems. The parallel-group model has been used to deal with different problems in parents and children. For example, a parent-child parallel-group helped improve the psychological functioning of depressed mothers, the mothers' caregivers, and children in Latino immigrant families. Therefore, the parent-child parallel group is considered an appropriate format for addressing the needs of cross-boundary families Two intervention studies evaluated the effectiveness of emotional regulation strategies on the parent-child relationship. The parental emotional management program increased emotional management strategy during the parent-child interaction, positive affect, subjective happiness, satisfaction with the parent-child relationship, and family harmony and decreased negative emotion among mothers of 6-8 years child in Hong Kong. However, this study did not include children in the intervention. A parent-child parallel emotional regulation group intervention improved the positive affect, resilience, and family harmony and reduced depressive symptoms among both parents and children. However, this study did not include the control group and had a small sample size.

   In sum, the present study aims to test the effectiveness of an emotion regulation parallel-group intervention on strengthening parent-child relationship and adaptation in newly arrived and cross-boundary families in Hong Kong.
4. Procedure We aim to recruit 200 parent-child pairs to participate in the study. The participants will be randomly assigned into one of the following arms: a) the relationship enhancement arm, which aims at improving emotional regulation skills among parent and child; b) the information provision arm, which aims at providing information about Hong Kong, such as education, community resources, medical care, employment, housing, and job-seeking. The ratio of the two arms is 3:2. Specifically, 120 and 80 parent-child pairs will be assigned to the relationship enhancement arm or the information provision arm, respectively. The two intervention arms will provide four 2-hour weekly sessions to the participants. The participants will finish the assessments at baseline (T1), immediately after the intervention (T2), and one month after completing the intervention (T3).