It's really difficult to get a special needs child out of nappies, but it is so worth the effort.
I created a routine with my children, starting slowly, letting them sit on the potty before bath time and early in the morning.
Usually the first time they go in the potty it's by accident, but once you show them, and praise them for what they did, they eventually get the concept!
I also let them wear 'big boy' undies, and if they wet them, I would take them and put them on the potty to make the connection.
It did take a long time for them to understand the concept, but so worth it.
I look at my 18 year old and I am gratefull everyday I don't have to change a nappy on him, it would be a huge struggle as he is in a wheelchair....and a very nasty business!
