Background and study aims? Parents with borderline personality disorder (BPD) can present parenting difficulties such as expressions of hostility, low sensitivity, and overprotection. These parenting problems are associated with adverse outcomes for the offspring, namely, borderline features, depression, internalizing and externalizing problems, and interpersonal difficulties. Intervention studies with parent(s) who have borderline personality disorder show promising results regarding the improvement of parenting skills and parent-infant relationship. However, very few assess their effect on child's mental health and development. This study aims to evaluate the acceptability, feasibility and preliminary effectiveness of a mentalization-based clinical intervention, directed to school-aged children of mothers and/or fathers with borderline personality disorder, to reduce child mental health problems, in the short- and medium-term.

Who can participate? School-aged children (5 to 12 years of age), with mental health problems (CBCL internalizing and/or externalizing scores must be T = 60 or above), and their mothers and/or fathers (> 18 years of age) with subclinical or clinical BPD.

What does the study involve? Mother and/or father with BPD complete an online survey and an online interview. If available, the other parent and the child's teacher complete an online survey, separately. The child completes an online task with a researcher's assistance. After, participants are assigned to an intervention group (one arm, pre-posttest study). Participants receive a mentalization-based treatment for children (MBT-C), starting one week after the pre-test. MBT-C is a psychological intervention designed to resolve the child's mental health problems and promote resilience by promoting the child's and parent's mentalizing capacities. It aims to be delivered by a mental healthcare professional trained in MBT-C. In this clinical trial MBT-C will be composed of: 3 assessment sessions; 12 individual sessions with the child, plus 6 parallel individual sessions with the parent with BPD; 1 follow-up family session, 3 months after the last session. Assessment sessions are composed of one family session, one session with the child and one session with the parent. Sessions with child are in-site and sessions with parent can be on-line or in-site, according to the parents' preference. Sessions have a weekly frequency and a 50-60 minutes duration each. One week after the last intervention session participants repeat the same assessments completed before MBT-C. In addition, child and mother and/or father with BPD complete separately an online satisfaction survey and an online interview to assess participants experiences with MBT-C.

The investigators expect that after receiving MBT-C the child's mental health problems will have significantly decreased and to obtain information on the feasibility of a future large-scale clinical trial and retrospective acceptability of MBT-C with this specific population.

What are the possible benefits and risks of participating? This intervention aims to resolve child's mentals mental health problems. Patients who do not meet the inclusion criteria to participate, do not consent, or withdraw from the trial will be offered a debrief and the possibility of being referred to individualized psychological support or other if needed. If child's mental health problems are not resolve by the end of MBT-C the same possibility will be offered. At the end of MBT-C mother and/or father with BPD will be referred to individual therapy, if needed and not already receiving. Intervention with BPD parents can present challenges, such as ambivalence within the therapeutic relationship (high idealization versus therapist rejection), and high drop-out rates. To overcome these difficulties, the therapist should be knowledgeable about BPD, foster a secure-based relationship with the parent and child, and maintain an empathetic stance. It is possible for parent(s) with BPD to maltreat or abuse their children, in which case child protective services must be contacted and informed. The same procedure must be adopted in case of substantial substance abuse.

Inclusion Criteria Child:

• have a mother and/or father with subclinical or clinical symptoms of BPD;
• present internalizing and/or externalizing problems (i.e., ≥60 in the internalizing and/or externalizing scale of CBCL 6-18);
• be 5-12 years old.

Inclusion Criteria Parent(s):

• have subclinical or clinical symptoms of BPD;
• have a school-aged child (5-12 years of age) with internalizing and/or externalizing problems.

Exclusion Criteria Child:

• unable to read/speak Portuguese;
• comorbidity with a neurodevelopmental disorder (i.e., autism spectrum disorder with moderate to severe impairment; moderate to severe cognitive deficit);
• doesn´t live partially or fully with parent with BPD;
• already receiving psychological counselling.

Exclusion Criteria Parent(s):

• unable to read/speak Portuguese;
• comorbidity with another mental health disorder that could interfere with intervention viability (i.e., schizophrenia spectrum or other psychotic disorders; substance-related and addictive disorders, except caffeine, tobacco and cannabis).