S.Q.
Great question and I think we have all been there! I think our comfort level with difference and the language we use makes a big impression on our little ones and in time they will become more comfortable talking about difference and about children with disabilities. For now, they notice everyone and everything and they have loads of questions- which is awesome!
From a very early age, like 2, I have used every-day encounters to point out to my two kids how all people are "alike" and "different" ; how we are "all built differently, and all perfect in our own selves". Like, I might point out how one child has blue eyes, the other green, but they both have brown hair. Or we use different colored crayons to match the right skin tone on drawings of us and others, talking about friends and family who have different shades and how pretty all those colors are. That people come in all shapes, colors and sizes.
I point out in books or on TV, "look, she uses a wheelchair. Or, do you know why that child might use a cane?" so that difference is more normal to them. I work with kids with special needs so they have always been exposed to kids who use walkers, wheelchairs or might look atypical, but but very, very beautiful to me. They see photos of kids I know and I talk about the child first, then the disability. I talk about how each child communicates: some use words, others signs or computers, but how that each child is a regular child with wonderful thoughts and exciting things to say and share with them.
I am also careful to say, "a child who uses a wheelchair" or a "child with autism" rather than a wheelchair-bound kid, or an autistic child, always putting the "child" before the disability.
My kids are at an age where they can understand how certain disabilities are caused so I explain that this was part of the child's genetic code or how their brain developed while in their mama's tummy. Your kids are a bit young for that explanation! But, as long as they know that the difference was not anyone's fault, just "how they are built" and that the child is still a wonderful child who just happens to use a wheelchair or a cane to help her get around.
You could rent videos from the library about kids with special needs or just start with how your child and her more typical friends are alike and different.
Good luck!