Participants who are randomly assigned to this arm, will receive traditional orofacial myofunctional therapy (OMT), consisting of interventions targeting orofacial strength, orofacial differentiation, breathing pattern, orofacial resting postures, and swallowing pattern.

Participants who are randomly assigned to this arm, will receive reduced orofacial myofunctional therapy (OMT), consisting of interventions targeting orofacial differentiation, breathing pattern, and orofacial resting postures.

Participants who are randomly assigned to this arm, will receive a sham treatment. This active control intervention will not include any of the exercises included in the experimental treatment arms. The sham treatment program will include exercises from an evidence-based vocal warm up program, with exclusion of the exercises that include active involvement of the tongue or lips. Furthermore, exercises regarding body posture (no focus on articulators), relaxation and voice onset will be included. This sham treatment program will be provided by the same speech-language pathologist with comparable motivation techniques as those applied in the real OMT programs. Patients and parents will initially not be informed about the actual purpose of this sham treatment. A debriefing will take place after the follow-up phase.

Behavioral interventions, like orofacial myofunctional therapy (OMT), target brain reorganization and plasticity by establishing new motor patterns or re-establishing old ones. The aim of OMT is to correct pathological function and resting positions based on the equilibrium theory. A service model that is often used in OMT is the model provided by Garliner, which includes a 10 week OMT program with a frequency of one session of 45 minutes per week together with home practice. Treatment sessions will be used to introduce and practice new exercises, to monitor home practice and motivation of the patients. Additionally, the participant will be instructed to perform the same exercises from the treatment session at home daily. This intervention will consist of (1) creating awareness, (2) establishing nasal breathing, (3) labial seal and lip closure exercises, (4) tongue posture exercises, (5) lip and tongue strengthening, (6) practicing a physiological swallowing act.

Measurement of dental occlusion characteristics (overbite, overjet, palatal surface area, palatal volume) will be based on 3D digital intraoral scans. The chief among which, anterior open bite, will be measured by vertical gap measurement (mm), i.e. the vertical distance between the incisal edge of the maxillary and mandibular central incisors when in occlusion.

Orofacial myofunctional status will be evaluated using the Orofacial Myofunctional Evaluation with Scores (OMES) protocol, a validated measure quantifying orofacial myofunctional behavior (with a lower score indicative of a more impaired orofacial function). Additionally, submandibular ultrasonography will be used to verify tongue posture at rest and swallowing pattern.

OHRQoL will be assessed using the Childrens Oral Health Impact Profile (COHIP) (a higher score is indicative of a better OHRQoL).