Answer: depends. :/
My dad died last year. DS was 2.5 and DD was 5 mo. My amazing MIL and not-local BFF drove 6 hours (separately) to support me and watch my kids.
Had they not done so, DH would have watched them, and I would not have had him for support (and he wanted to be there anyway, he liked my dad).
There's no way my kids would have done well sitting in the funeral, which was also a full Catholic Mass.
But had they been older, say 5 and 7, I would have expected them to be there.
I attended my grandfather's funeral at 7 and my grandmother's at 11.