I'm going to offer costumes to my neighbor for world book day. She suggested to her daughter to pick a character who wears normal clothes
Did she have a problem with the other outfit being too wild?
She's only making this argument in order to challenge my belief in costume.
Despite the mother's strange reasons, it's still her prerogative to make sure whatever her kid wears is appropriate. So it's helpful and nice of you to give out ideas for clothing she would like, you never know if she develops a nice style after this.
She can wear whatever. There's no character that has anything remotely similar to what I'm wearing.
You gave your input and suggestion but unfortunately they turned it down, which is ok. You should be satisfied with offering your help and move on. If they don't appreciate your opinion, ignore them.