Parents are asked to track the time they spend working on the VR homework exercises on a VR tracking sheet on a weekly basis. They are asked to fill out how long and which days they practiced. The number of days practiced will be divided by the total number of days in the week. This yields a percentage of VR completion per week.

Specifically for this study three questions were designed to measure the level of immersion children experience while practicing with the VR. Children are asked to rate their immersive experience on 4-point Likert scale ranging from strongly disagree to strongly agree.

Semi-structured interview for children and adolescents between 8 -18. The interview is suitable for both children and their parents. The SCID-5 Junior consists of 17 modules to classify DSM-5 disorders. The SCID-5 Junior is used in standard clinical care, to check for additional anxiety disorders and disorders that might look like SM. The SCID-5 Junior will be administered in the same manner as in regular clinical care, meaning that parents only report on the modules: anxiety, autism and obsessive-compulsive behavior, unless there are indications that suggest the presence of another co-morbid disorder.

The FSSC-R / VAK 4-12 is a self-report questionnaire that consists of 80 items (0-240; higher scores indicate more anxiety symptoms). The items are descriptions of objects or situations that may cause anxiety. The questionnaire can be administered verbally for children who are not able to read. Children are asked to indicate for each item whether it makes them feel 'not anxious', 'a little bit anxious' or 'very anxious'.

The CBCL aims to quantify problem behavior and competences of children and adolescents. This questionnaire with 113 items (0-226, higher scores indicate more emotional and behavioral problems) is used in standard clinical care for routine outcome measuring. The questionnaire is filled out by the parents or caregivers of the child. Parents (both mothers and fathers) indicate on a 3-point scale whether the behavior was present in the last few months. The behavior can be classified according to different scales: Internalizing, Externalizing and other problems.

(C-)TRF is the teacher's version of the CBCL. The TRF has 113 items (0-226, higher scores indicate more emotional and behavioral problems) and is included in standard clinical care for routine outcome measuring. The questionnaire can be filled out by a teacher. The questionnaire gives insight in the child's behavior and competences at school as evaluated by the teacher.

With the Dutch OBVL, parents are asked to evaluate statements on how they experience their child and rearing, how they relate to their child and how they are feeling. On a 4-point scale parents indicate to what extend they agree with each statement (34 items, 34-136, higher scores indicate less parenting burden). The OBVL consists of 5 subscales: problems in the caregiver-child relationship, problems with upbringing, depressive moods, role restriction and health problems.

The questionnaire evaluates the level of family accommodation related to the child's anxiety and the level of distress the child experiences. The FASA consists of 13-questions that are scored on a 5-point Likert scale ranging from "never" to "daily". The first 9 items form the total accommodation scale (0-36, higher scores indicate more family accommodation). The subscale assessing the level of distress (5 items) is scored on a 5-point Likert scale ranging from "no" to "extreme." The FASA is included in standard clinical care for routine outcome measuring.

The CGI is a 2-item, clinician-rated scale used to assess global illness severity (1-7; higher score indicates more severity), over-all improvement (1-7, higher score indicates less improvement) from the start of treatment.

The Outcome Rating Scale (ORS) is used to monitor children's, young people and their families' or caregivers' feedback on therapeutic progress. The ORS is a 4-item session-by-session measure designed to assess areas of life functioning known to change as a result of therapeutic intervention. These areas include: personal or symptom distress, interpersonal well-being, social role and overall well-being (for each item 0-10; higher scores indicate better functioning)

Children are asked to indicate the level of anxiety, using a visual analogue scale, after completing a VR session and after individual therapy. The child can indicate whether they feel none, mild, mild - moderate, moderate, moderate - high or high anxiety.

A qualitative evaluation will be made after the intervention. During the interview the child will be asked to answer a few short semi-structured questions on their experiences with the VR homework exercises.

The researcher will schedule a 15-minute semi-structured interview to ask parents about their experiences during the VR sessions. The interview contains questions re-garding the practical use of the VR headset, how often parents have practised, the level of anxiety their child experiences while practicing and whether their child likes working with the VR homework exercises. Lastly, parents are asked whether there have been any special events that might have had an influence on the child's SM symptoms or the VR sessions.

A qualitative evaluation will be made after the intervention and after 3 months of follow-up. Both parents or caregivers are asked to share their experiences with the VR homework exercises and their vision on how the use of the VR homework exercises has helped their child to gain the confidence to speak at school.

After the intervention, therapists are asked to share their perspective about the use of the VR homework exercises in addition to the standard treatment. They are asked to identify what factors, such as the relationship with parents or specific characteristics of the child/family contribute or hinder successful usage of the VR homework exercises.

This questionnaire evaluates adaptive behavior on five domains: communication, daily living skills, socialization, motor skills and maladaptive behavior. The questionnaire is suitable for both parents and teachers. Only the domain communication will be assessed as the presence of speech and language problems is often linked to SM.

For the purpose of this study, the questions were rephrased to turn the original questionnaire into a parent-report. Parents are asked to evaluate their child's immersive tendencies on a 7-point likert scale, ranging from strongly disagree to strongly agree (10 items, 0-70, higher scores indicate more immersive tendencies).

This questionnaire contains questions with regards to background information about the child and his or her family. Questions concerning sex, age, ethnicity, family composition, parental work situation and education level and medical history (including treatment background and earlier diagnoses given to both parents and child) are included.