The ePub ebook standard in principle includes DAISY, but none of the current ePub readers support it so far as I know. I think you may still be better off with a mainstream device, because they are cheaper and have in general wider content availability. You may be able to get free ePub ebooks, and perhaps even free Nook Color kids ebooks (see below), via various agencies because of your child's cerebral palsy.
Two good choices would be the Nook Color or the
PocketBook IQ 701. The Nook Color has support for read aloud in Nook-specific children's ebooks (
Nook Color Kids). The PocketBook IQ is an Android tablet optimized for reading, and so it likely supports
android-daisy-epub-reader-project for DAISY ebooks. Both of these support Adobe ePub ebooks (e.g. from the Sony ebook store of Books on Board and from many public libraries) but only B&N supports ePubs from the B&N store.
You can try out the Nook Color at any B&N store. For the Pocketbook, you may want to ask in the PocketBook forum here about android-daisy-epub-reader-project (i.e. have someone with an IQ confirm that it works).