I've just looked over the bits of dd1's form that I'v already done, and I've filled it in as has been suggested here - lots of emphasis on how much help she needs (I find this quite hard, as dd1 is my eldest child, so comparing her to "normal" is not that easy - everyhting I do for her has been natural extension of a situation over the last 3 1/2 years...)
tbh, I am struggling woith the form for the same rreasons as you - not wanting to make out that dd1 is worse off than she is, but I have recently come to the conclusion that it doesn't matter what the people who read the form think about dd. I know that she is not as bad as it comes across on the forms, as does everyone else involved with her. But (and it's a big but), she is a lot more work (even in the hands-on, supervisory way) than a NT 3 year old. There is nothing she can do for herself (even things she can do, she doesn't want to). It is just like having a large, amiable baby around.
Dd1 is very passive (as I think, you said yor ds is) but this can be as much of a negative as a positive. It means she will sit doing nothing until directed. It means she will not eat until told to. Or drink until reminded. Or get up to get a book/toy/puzzle. Once she has the book/toy/puzzle, she will sit there with it until reminded to do something with it. And then the next action, and the next.
None of this is any more difficult than parenting an NT child, but it is a lot more constant and very wearing. and so, if this means that I have to emphasise dd's negative passitvity for the DLA form, then so be it.