My youngest DD has cerebral palsy and first went to WDW when she was 11 months old. She has been to WDW at least once a year since then and is now a young adult.
It's really hard to give specifics because people with cerebral palsy's symptoms can have a very wide range. It's very hard to say what the end result will be looking at the current symptoms of an infant.
My DD has mixed CP, mostly spasticity affecting her arms and legs, but her muscle tone varies between rag doll limp and Barbie doll stiff.
She can't stand without someone holding her up and can't walk without being totally supported in a gait trainer, which holds up her trunk and secures her in a standing position. She is able to propel a manual wheelchair, but not for any great distances. She can drive her power wheelchair with a joystick. She understands, but is not able to speak.
She has a friend with athetoid CP who has a lot of slow writhing movements of her arms and legs. She does not have control of her arms to drive a wheelchair and uses controls in her headrest to drive her power wheelchair.
On the other end of the spectrum, DD had a daycare provider at one time who had CP. One side of her body was affected and she had limited use of one arm. Her leg on that side was also affected, but the only sign was that she walked a little slower when she was tired. There are also people who have no noticeable signs of CP, just some weakness or tightness in the affected areas or a bit of clumsiness.
So, as you can see, there is quite a large range of possibility. I'd suggest the website of United Cerebral Palsy as a good website to start with:
http://www.ucp.org/
They have been in existence for a long time - I think more than 60 years -and have local affiliates to the charity as well as a national organization.
As far a 'help' from WDW, since your child is Under 2 and will still be under 2 when you travel, it is really hard to know what sort of assistance she might need. The biggest help for a child that young is being able to bring a stroller into line and have it treated thevsame as a wheelchair.
This does not mean shorter waits - most of the lines are totally wheelchair accessible. But, strollers are not normally allowed in lines and you would otherwise have to carry your child. You will find more information about using a stroller as a wheelchair in post 6 of the disABILITIES FAQs thread. That thread is near the top of this board or you can follow the link in my signature.
The other big help is the First Aid station in each park. If she has equipment that needs to be stored, a place for treatments you need to do, or she just needs a good stretch, you can do that in First Aid. Those services are free of charge.
My best advice is that traveling with a 15 month with CP. Is not that different than traveling with a 15 month without CP. There are many attractions that infants can go on and you will be able to hold her or seat her next to you and support her. A child that age is pretty 'portable' and you should have little difficulty boarding any of the attractions that infants are able to go on.
You may find you do certain attractions for 'unusual' reasons. My DD liked spinning rides like the Teacups, but we 'used' them when her muscles were getting tight because the spinning motion relaxed her muscles and made her looser. While she was loose after the ride, I would sit down and place her on my legs with her head at my knees, her trunk coming towards my supported on my legs. Her butt was at my waist and her legs were extended straight up my trunk with her feet at my shoulders. A bit strange, but a very effective way to stretch her legs.
Feel free to ask as many questions as you need, especially as you know more about her condition.