I have been asked to go to a underwear party should I go ?

My auntie has 1 child( my cousin) for his party he is having a underwear party so everyone come in boxers or briefs and that's it my cousin is going to be 8 so their are going to be about 30 kids boys and girls they have asked me to be their but in my underwear ( briefs) but as I am 16 and in puberty I don't want any b o n e r s or things like that would should I do any help pls