ask at your parish what customs are
in general, Baptism, whether or not it occurs within Mass or at another time, is a public act of worship and open to anyone (space permitting of course). So you may want to weight that. If baptism are done in your parish during a Mass that is usually crowded, ask if it is prudent to limit the number of family members present, or if they should get there early, sit in a particular place. If you will be having some type of celebration at home, you could issue invitations for that, and just note that the celebration is in honor of Child’s Name, who will be baptized at 9 am on that day at St. Mary Church. the baby won’t care so the party is for the family.
If there are going to be 40-50 babies baptized at one time (common in this parish) that means each family will have one pew reserved for the parents, godparents and child and everybody else sits where they can find room, that does not leave a lot of space, so that might be a factor in your decision
as for what to wear, find out in your pre-baptismal class if the children will be baptized by pouring or immersion, or if you have a choice. Most parishes give a symbolic white garment so although white is the traditional choice it is not required. If you have to change the baby anyhow, just bring him in a onesie and dress him after baptism. If he is older just whatever you would put on him for church or any dress occasion. If the elaborate gowns, suits, whatever are traditional for your own family or culture go for it, but there is no church requirement per se.
You’ll wear yourself out trying to make them understand, so if it is brought up, let it be known your decision is made.
