Got married last year and DD was our bridesmaid - 24 months.
You need someone to be a dedicated person to just her. One of your family or friends will have to not drink (surely someone can give up alcohol for the sake of one day???) If it could be one person all day it would be best (buy em a bottle of champagne or something as a thanks) because it is disorienting.
My DD was thrown by mummy in a big dress with fancy hair, and worst of all, no glasses... she wouldn't go near me for half the day! Have either the dedicated person look smart, or have a bridesmaid ready to carry her down the aisle should fear of mummy set in
. I didn't have it planned, but my friend (the carer) was conveniently in a suit the same colour as the wedding :)
If possible, can anyone have her in the morning whilst you get ready - she can meet you at the ceremony and you don't have the stress of baths, sticky fingers on dress etc.
A comfy pushchair is a great suugestion - both page boy and bridesmaid were conked out snuggled up with blankets by the end of the day.
There will be some time between the ceremony and the do while you disappear for photos (I should think) - plan for someone to take her for a drive, either to a park to let off steam, or to sleep during that gap.
A change of clothes is handy - bridesmaid dresses are normally manmade fabrics and can get uncomfortable... A pretty cotton dress for the evening will make her feel more comfortable.
Oh, and be prepared for a toddler on your lap during the meal
.
It can be an incredible day with your child, but planning is essential - you need to be able to focus on you.