DD's 1st trip was at 5-1/2 months. Her 2nd trip was at 12 months. She enjoyed everything because at that age it is the lights, sounds, colors, etc. that babies like.
My advice is this...
Go at your child's pace or else suffer the consequences!
Try to keep as close as possible to your child's usual schedule - wake up time, nap time, meal times, bed time, etc. It always upsets me to see parents dragging around half-dead kids at 10pm because mom or dad just has to see Illuminations.
Make this trip about your child. Take your time. If you don't get to every attraction or every show, oh well. There's always next time.
Take frequent quiet breaks during the day. Yes, even in the parks, there are areas that are away from the crowds, out of the sun and relatively peaceful. You just have to look around for them. Its very easy for a baby/toddler to hit that sensory overload point at WDW, then forget trying to get them to nap when that happens.
Have a great time. Our daughter, now 7 and with 9 trips under her belt, loved WDW at that age. Your kid will too.
Steve