My husband will be doing the antlers and brown shirt for his costume!
5 year old will be Olaf. Since he has sensory issues and won't do costumes, we have to do a regular t-shirt for real Halloween too. I found an Olaf t-shirt that is Olaf's face. But if you're crafty, you could probably create something easily. Actually, even I could have probably created something but I was crafted out after doing Anna and Elsa.
8 year old is Elsa. And I'm Anna. Both are homemade costumes, but more inspired than exact, as it's going to be hot!
I agree with being Olaf.