It does not matter if you are deaf or blind, or suffer from a severe physical disability that limits your movement. Apple devices are the right choice for you, because they were designed for people with any disability.
The best iPad for your child is probably the entry level iPad offered by Apple for only $329. You could spend $1,649 for the top-of-the-line iPad with 1 terabyte of storage, but that's almost certainly not necessary because you can do the same things on both devices.
An iPad with Wi-Fi + Cellular connectivity will allow you to access the internet anywhere cellphone reception is found, whereas an iPad without cellular connectivity must rely on Wi-Fi or a HotSpot to access the internet. How you use an iPad will determine which one you should purchase.
When comparing the best iPhone with the best iPad available today, the iPhone offers a superior camera by far. However, when considering what the camera for each device is designed for, both the iPhone and iPad offer an excellent camera experience for its intended use.