## 1 Introduction  
Children with complex communication needs (CCN) may have a developmental disorder such as autism-spectrum disorder, cerebral palsy or Down syndrome, or they may have an acquired disorder as a result of a traumatic brain injury or stroke, which results in disordered speech abilities. Children with CCN fall within a spectrum, as some present without intelligible speech whilst others have developed minimal speech, but cannot primarily rely on their speech to communicate [1]. When someone cannot rely on their own speech to communicate, they often look to other techniques to provide them with an alternative means to communicate. One such alternative is to use augmentative and alternative communication (AAC) devices. An AAC device with the capability for speech synthesis is referred to as a speech-generating device (SGD) or a voice-output communication aid (VOCA). The use of SGDs has been shown to increase the quality of life for individuals with speech impairments [2]. Without the ability to communicate, individuals may withdraw from social interaction, and even from interaction with their own family. This is often compounded when they are required to use a synthetic voice that sounds robotic, sounds like someone from a different geographical or social background, or someone of a different age or sex [3]. In fact, it is not uncommon to see a nine-year-old girl using an adult male voice or several children in a classroom using the same voice [4; 5]. Speech-synthesis technology has traditionally focused on adult speech. Developing synthetic child speech could be considered more challenging for researchers, and even more so for children with CCN who make use of low-resource languages. Despite these challenges, researchers have begun exploring new techniques and methods for natural and intelligible-sounding child-speech synthesis. Communication is essential to learning; without access to functional communication, children with CCN are often restricted from participating in the classroom [6]. This may result in fewer instances of intentional communication, minimal language and literacy development, and poor opportunities for socialisation [1]. As a result, their potential is often underestimated. This has repercussions for other areas of their life, such as accessing healthcare, participating in family and community activities, as well as engaging in future employment [7]. Due to the importance of communication in our social, educational, professional, and personal lives, it is essential that the speech output of SGDs be as intelligible as possible. Using natural speech as the benchmark, _intelligibility_ is defined as the accuracy with which an acoustic signal is conveyed by a speaker and recovered by a listener [8]. A synthesised voice with high intelligibility is thus required for the acceptability of the SGD, not only in terms of it providing the service for which is was designed, but also in terms of the positive attitudes towards the voice and the social interaction with the user [9]. In contrast to intelligibility, in which the speech signal is extracted from the context, _comprehensibility or_ understandability is the degree to which speech is understood when combined with available relevant information (i.e., linguistic context or conversational topics) [8]. Speech naturalness and pleasantness could be considered important components of comprehensibility. _Speech naturalness_ can be described as how well the speech matches a listener's standards of rate, rhythm, intonation, and stress. Therefore, speech naturalness determines how natural the speech sounds to the listener and is compared to the speech they regularly hear in their immediate social environment [10]. Using a voice with a low level of naturalness, such as a robotic voice, does not match the human that it represents.