Wow really?! I guess she thought she was doing a good thing, but that's clearly not for everyone.
I knew of a lady who's young son sadly passed away, it was expected and she'd been nursing him at home. She kept him in his room until the funeral so she and his siblings could sit with him and talk to him, I was a bit shocked initially to hear that but they found it a very positive experience. It worked for them, they were happy with that.
I think how we handle the passing of loved ones, funerals etc its all very individual and I suppose this is what makes it so difficult. Of course, there is a huge part of you that wants to protect your child from any of this, I know you can't, but you still want to. I know his mum wanted to protect him from anything like it, we'd had a conversation only a few weeks ago on the subject after him having nightmares about dying.
My husband feels under pressure from his ex's mum to take him, I just said you gotta do what you feel is right for him, no one else. He wants to do something at home for him, he feels he will be able to say goodbye properly like this as there will likely be hundreds of people at the funeral. We also worry about him asking about where's mum going, because cremation isn't exactly the nicest thing to explain. I had to explain it all to my son but he was almost 9 at the time so he knew there was no possibility of his dad being alive and therefore being burned alive. But then for my step son only being four, his little imagination may run wild.
To be honest, I think the only time we would know if it was right for him to go would be on the day, then it's too late if he reacts badly to it.