Hi op, based only on the further info you put your child's needs would come under mobility and so you wouldn't qualify for dla until they are three.
Mobility allowance is one component of dla it is a weekly sum that can be exchanged for a car, you don't have to drive you can just take the cash. It's for anyone that can't walk or has ristricted walking/moving around from 3 years old.
To claim the care component of dla which is the only part you can claim for under threes, you would need to be providing extra care not related to mobility (moving about). So things like feeding, getting dressed, toileting getting in and out of bed. But as these are things that all under threes need care for, it's very hard to successfully claim. I know blind children and children with downs syndrome that have been rejected.
On your application you need to consentrate on time you spend doing things like physio or speech and language therepy and any medication they need. Think of anything which is different for you than it would be for a regular 18 month old, like extra help and supervision in the bath, extra help feeding or special diet.
In regards to aids speak with your physio. Sometimes you have to ask they won't offer. Our physio provided standing frames, benches and seating. Also get a referal to children's occupational therapy, they can provide adapted high chairs, buggies, bath seats any sort of aid you need to make caring safer /easier.