NTA

There’s no good reason she couldn’t bring the child along, or, make the difficult choice of forgoing the funeral and OP isn’t neglecting his parenting duty by not stepping in

Their son isn’t having a health emergency, he wouldn’t be harmed by traveling with mom and gathering with family and he isn’t old enough that there would be a disruption to his education. If any of those things were true, OP would be at obligation to step in for his son’s benefit.

What the ex is asking for is a favor to reduce her personal hardship of having to seek out childcare, travel with child to the funeral, or forgo attendance. It’s her family emergency during her time with their son. She needs to put her son first, even if the options available to her aren’t ideal