My nieces have attended funerals or visitations since pretty young. My brother and sis-in-law are always ready to talk about it with them. The girls handle it pretty well. I think it's good for kids to go. Your kids, like my nieces, are pretty sharp, and you have a good relationship so if Thor wants to talk to you I think he will.
It is also something that will mean something to Fred, if not yet, then later.
As an aside, my youngest niece was probably 3 when she went to a visitation with the family. Later she was asked what she thought about it, and she shrugged and said "we got there and the guy was already dead." Some visit!
