Hi Jennifer,
First of all, let me say how sorry I am that you have lost Abby. I am sending prayers your way.
My daughter was 7 months when she passed away and her older sister was 3 years old, just like yours. We decided not to take her to the visitation (which was open casket), because we were not sure we wanted her to see the body. But we very much wanted her to be a part of the funeral services. She even had a part in the graveside ceremony. I had the florist make up a little nosegay for her to lay on Emma's casket at the end of the service. I don't know if she remembers any of the funeral now, but she is there in the pictures, and that is important to me.
I hope that helps you make your decision. I remember asking our pastor about it and he pretty much said, whatever you decide will be fine. He felt like that was our decision and there were no right or wrong answers. I think if she wants to go, then it would be good to let her. It will possibly help her process some of it.
Just a side note. Be prepared for her to ask you questions a long time later. Kids process grief differently at different ages, and apparently they will go through different stages of it as they get older. So we had to re-explain things and answer more questions as Elise got older.
Miriam