Obstructive sleep apnea (OSA) is a prevalent medical condition with important implications for overall health and quality of life in both children. Therefore, it is important to treat OSA early and effectively. Children with Down syndrome and Prader-Willi syndrome have many predisposing factors for OSA, including mouth breathing, narrow upper airways resulting from craniofacial abnormalities, and generalized hypotonia, which increases UA collapsibility and multilevel obstructions. Adenotonsillectomy is the first-line treatment. Unfortunately, up to 55% of children with Down syndrome and up to 79% of children with Prader-Willi syndrome suffer from residual OSA after adenotonsillectomy. Therefore, exploring other treatment options for these children is an interesting and relevant avenue for research. This study will evaluate the effectiveness of orofacial myofunctional therapy as a treatment option for children with Down syndrome or Prader-Willi syndrome and obstructive sleep apnea. Orofacial myofunctional therapy consists of a set of oropharyngeal exercises to correct abnormal orofacial functions and strengthen upper airway muscles that are involved in maintaining airway patency. Both objective and subjective/patient-reported outcomes are collected to obtain a comprehensive understanding of the potential of orofacial myofunctional therapy as a treatment for OSA.