to my knowledge there aren't any specific AAC devices for tunnel vision. If it's "only" tunnel vision, she could have the sizes of the icons modified to fit her remaining field of vision. It would make it slower but probably more doable.
For people with cerebral palsy the most common cause of blindness is CVI, which has some ways of making traditional AAC like tablets work. For example, not using visual icons which only result in visual clutter and memorizing the positions of phrases/words/letters (you can read a bit about it here) instead. This is easier if she only uses a relatively small amount of words, but it can be a very time-consuming process to learn how to use it effectively.
The most basic way would probably be print-on-palm, which is basically what it sounds like (there is also a braille version where instead of letters it's braille dots). It's time-consuming and requires at least some amount of finger function (so it might or might not be actually accessible for her) but it's a frequently used communication method between a DeafBlind and a seeing-hearing person despite it because it only requires that the other person knows the alphabet.
There are also braille-based communication devices, but all of them require braille knowledge (which not all DeafBlind people have) and many of them require decent finger function (which not all DeafBlind people have, especially those with neurological conditions). But you can see some of them in this video.
If any followers have suggestions, feel free to leave them in the notes.
Seconding Mod Sasza's input on what will make an accessible AAC device for her to communicate with. Honestly, I'd put more attention into how she receives language.
Cochlear implant users can have a variety of speech understanding ranging from "mostly understands speech when it's clear and with the aid of lipreading" to "with the cochlear on, just hears static/noise and can't understand the sound." Consider whether she is bilaterally or unilaterally implanted, and how that impacts her receptive language skills (bilateral implants are generally better for oral language skills). Can she not speak because of her CP, or because her Deafness makes it hard? If she can't speak because she is profoundly deaf, she probably can't understand speech either.
Tactile sign language, especially ProTactile, relies on some motor strength of the recipient, and for ProTactile access to the back, so if she's a wheelchair user full ProTactile won't be super possible. Tactile sign language requires motor strength because your hands need to feel other people's hands. Non-tactile sign language requires eye-tracking ability and the signer remaining in her field of vision with high-contrast clothing and probably a slightly exaggerated signing style.
A braille reader could be a solution, or large high contrast text on a screen, but they both require someone to type out what is being said. Using a braille reader will additionally be dependent on her ability to move her fingers across the reader, and she may have to adjust the speed. On-screen text requires some ability to eye-track, which can be exhausting for both blind people and people with severe cerebral palsy.
One possibility, like Mod Sasza mentioned with print-on-palm, is a tactile sign system or alphabet signed on her body. This is somewhat risky because it requires touching her in a way that she doesn't have control over, but she could read the manual alphabet pressed into her hand even if she can't keep her hands on top of someone else's for tactile sign language. For a method like this, having distinct and frequent check-ins about touch is vital to prevent abuse.
One more thing to consider in terms of a communication system for your character is whether she is language deprived.
Her deafness could be caught at birth from a lack of auditory brainstem response, but she couldn't get cochlear implants until a minimum age of 9 months, during which time she will have little to no exposure to auditory input. Deafness that is present from infancy but still has auditory brainstem response at birth can go unnoticed until age 2-3, if not later, when delays in speech and auditory response become more obvious. Still, these children are often tested for developmental disabilities like autism before they are assessed for deafness.
Since she has tunnel vision, it may be detectable through pupil response to light, but it will likely be fairly difficult to assess her vision at a young age. She might not even be diagnosed until she has the functional communication to express that she can't see. Even then, people who are low-vision from birth may not realize that their vision is different from sighted people's.
Similarly to many forms of deafness, cerebral palsy is often not diagnosed until age 2. More severe cases can be caught earlier, but it still takes missing major milestones to be diagnosed. Additionally, her CP will likely mask her DeafBlindness. Low vision in children is typically assessed through hand-eye coordination or difficulties with controlling eye movement, both of which could be attributed to cerebral palsy. Deafness, if not caught at birth, could be chalked up to CP as a lack of motor control to turn in the direction of sound, speak, or follow verbal directions.
The startle reflex (uncontrollable muscle contractions at sudden sensory stimuli) staying present beyond a few months of age is common in CP, so lack of a startle reflex could be a sign of vision or hearing problems, but not everyone with CP, especially if they do not have a hypertonic (spastic) subtype, will experience startle reflexes. Undeveloped reflexes are a symptom of some subtypes of cerebral palsy.
In summary, it is highly likely that at least one of her disabilities, and especially her tunnel vision, may not be caught until much later in her life. This puts her at high risk for language deprivation. Otherwise abled Deaf and especially DeafBlind children are very likely to be language deprived, and having another disability that impacts her expressive ability will add to this.
Not having early access to language means she might struggle to understand systems that rely on spelling. Deaf children don't learn well from phonics-based language models, and many have issues spelling. Language-deprived Deaf children struggle even more--sign languages are often their first, if not only, language, and acquired in late childhood to teenage years. Language-deprived children struggle to express needs and often have poor emotional control.
Without any additional language processing disabilities like autism, language-deprived children may find a visual or symbol-based language more accessible than a written one. Spelling often feels meaningless without a basis in another form of language (tactile, visual, or auditory).
Whichever form of AAC ends up working for her depends immensely on her receptive language skills--how she understands language and the world around her.
