Why is clann wrong here?
It shouldn't be, since clann can only mean 'kids' when used as family; you can't use it to refer to brothers and sisters and parents.
Clann can also mean “descendants”.
True. I was speaking more how you couldn't use it to talk about your brothers/sisters and parents. Edited to clarify.
Why is gasúir wrong? That's what they say in connemara...
Thats mostly used as a local term normally describing youths or messers. In Connemara it wouldnt be wrong but to other Irish speakers elsewhere they may misunderstand you