Emily the question that they pose themselves is 'what can dc do on a bad day' and they mean without aids such as loads of prompting that most kids dc's age don't need.
I have ms and on a good day I am totally self caring for washing dressing etc.
On a bad day I do not have the energy, or my limbs just won't co-operate. If I could predict exactly when the bad days would be, then there would be an argument to reduce the amount, but I can't.
Your dc does need help with washing and dressing - what would she be like if you did nothing to help for a month? How clean and hygienic would she be?
She does need help with mobility - what would happen if you weren't there to encourage her when she becomes anxious? It sounds like she could be in danger without your help and encouragement.
You don't pay taxes by choice, and the flip side is that you are entitled to the help.
Don't feel guilt - the under funding of people with needs is not your fault - the MPs with their double digit pay rises, tax breaks for rich people, and tax avoidance and evasion should be playing their part before you start trying to reduce what you are entitled to.
Just think of all the ways you will be able to help her now that were not available before. For a start you will have a fund to replace the belongiings that keep getting lost....