For a toddler girl's birthday party, pick a dress or skirt made from double-layer muslin cotton: soft against her skin, breathable if she's dancing or running around, and forgiving if cake or grass gets on it. A twirly dress works well for photos, while a skort keeps her comfortable if there's a lot of climbing and jumping.
We make double-layer muslin cotton clothes for babies and toddlers, and birthday parties are exactly the kind of day our fabric is built for: lots of running around, maybe some cake, maybe a nap in the car afterward. Here is what we would put a toddler girl in for a party, and why we picked each piece.