2.1. Design
This study was approved by the European University of Madrid Ethics Committee (CIPI/20/153). The trial was registered on ClinicalTrials.gov (ID: NCT05611554). Randomisation was completed by the Principal Investigator using an online service and randomly sized, permeated blocks to each arm (ratio: 1:1). The trial was a single-centre RCT with two arms, with participants assigned to: (1) psychological flexibility programme for parents (intervention group) or (2) waiting list (control group). Participants were assessed at baseline (pre-treatment), at the end of the programme (post-treatment), and 3 months after the end of the programme (follow-up). Participants on the waiting list condition remained in the study after assessment and received psychological intervention.