How would you explain to your child that his or her younger sibling is a special needs child?
Funny story for you. Our girls are only 8 months apart, both preemies. The older one has no disability and has never known her sister with CP any other way. The younger girl cannot walk unassisted and also has one arm that isn't fully functioning.
Over the years we have, of course, made many visits for PT and OT and the older girl went with us. We got a walker and a wheel chair, and the older girl saw all this, but she never questioned us about her sister.
When they were both 4 years old the older one finally asked me why her sister didn't do something and I just looked at her and then it dawned on me that she truly saw her sister as just another kid, but maybe a lazy one.
I handled it by asking the older girl how old she was and how old her sister was. 4 she said. I asked her if she had noticed that her sister's legs were different from hers. Yes, she answered, and then added and I don't think that one arm works good either. So, I said, can you see that your sister is different and always will be. Yes, she said "but she's still a great sister and I guess it's OK if she doesn't walk."
Over the course of the next couple of weeks, with both girls in the room I explained more about her sister's problems. The girls are 7 and 8 now and there have never been any more questions about her condition.
So I guess my answer is that we never made a big deal out of the differences and let the girls become aware on their own. All questions have been answered honestly and with as much detail as they could grasp at the time. Perhaps it was easier for us since the younger girl has no mental impairment and has always been able to enjoy the same books and TV and toys as the older girl.
As they have gotten older they have learned that one child likes to be outside and the other doesn't. One has a bike and one a computer, they are very different personalities, but have adjusted very well to each other's differences.
There are actually books out there on how to explain a disability of a child to other children. You might try this link which has several books on different disabilities, which range from learning disabilities to food allergies.