I am sorry for your loss, r3.
I think if it were me, I'd take dd1 along. But then she is not so severely delayed, and we would be able to talk to her about the day afterwards, and use it to remind her of what had happened when she wanted to know where the person had gone.
What is the rest of your family like, wrt accepting dd1? Are there any members who are likely to comment/getupset/think it inappropriate?
What is your mum likely to think? If this was about your dh's father's funeral, for eg, (assuming similar relationship), what would your mum advise you to do?
Forus, as part of making snese of it all, we would be making a photobook for dd1, so we could talk through the day afterwards.
And there would no doubt be lemon-suckers in my family, but I would not give a shit (uness it was likely to upset key people, ie your mum)
do you think it would be important for your dd1? If only so you would have something to say (whether she understands it or not) when she asks for your dad.