I take it you are a guest at an infant baptism.
You might asks your hosts, the baby’s parents, what THEY will wear. Don’t ask the baby. He or she does not care, and might be immersed. If this is the case, he or she will be wearing NOTHING except a diaper until after the actual baptism, and you really don’t want to show up wearing that.
A lot of Catholic churches are pretty casual nowadays (sometimes to detriment). Barring flip-flops, tank tops, shorts, tight clothing and revealing clothing, you could easily get away with a pair of pressed khakis, a button-down shirt and a decent pair of shoes.
Dress in the khakis and button-down. Put a tie and a blazer or sport coat in the car and take a peek how people going in are dressed as you park. If they are all at “khaki” level, then leave the tie and coat in the car. If the guys are wearing ties, well, slip on hte coat and tie.