Our key saving grace at social/public functions has always been inclusion somehow. Maybe there us an extra special job that only your child can do. Maybe its a more front and center inclusive task or a more in the back off to the side task. Either way giving them something special to focus on is a wonderful way to curb those outside influences from resulting in a meltdown.
One day at church while in the middle of my sons full blown meltdown our priest simply smiled continued on and afterward as I tried to apologize he said "we're all God's children and want to be heard how wonderful to hear him call out to God and give praise.".

Enjoy the day and try not to stress, it will be wonderful in God's eyes.

If you do decide to have her in with you instead of in Sunday School here are some things that have helped us. From birth on up our boys have attended church with us. We bring plenty of snacks/non spillable drinks, plenty of quiet activities for them to do (we do not do electronics in church but it is an option w/ headphones), we allow them to sit on the floor or kneeler. We've found that sitting on the kneeler works best because then they can use the pew as a work space. We have two children with ASD so seating arrangement was key for us. We placed our non-runner on the end, then myself, then our runner, then my husband, and finally our NT child. Quiet activities included; coloring books, books to read, blocks, cars, super heroes. My two were 4 (functional level of 2 1/2) and 6 when my youngest was baptized and we are Byzantine Catholic so our mass is very long (never shorter than and hour, usually closer to 2 hrs)and they both did fine. Good Luck and don't be afraid to use the nursery.

Personally, I would have go to Sunday school. She too young to remember being there, it won't mean anything to the baby and it'll just make you a nervous wreck. You should be able to enjoy the baptism. Besides, she'll be more comfortable in her routine.

Thx for your feesback. She had a little tantrum abt something right before the service so we made a game time decision to have her stay in the nursery and our son's baptism went smoothly. Afterwards she hung out at the reception for 5 mins, then beelined for the parking lot and said "I want mommy's car. I want to go home." Not the life of the party, I guess ;)