Please tell DD to not worry about getting anything too formal. Most of the congregation will show up in their normal meeting attire (modest skirts/dresses for the women, and either a suit or shirt/sweater with slacks for the men). I've seen people in jeans (and women in pants) at memorials. It's not common, but people would probably just assume they weren't JW's. No biggie.
Regarding the baby, there is a room behind the main auditorium, sometimes referred to as the "crying room" (could be a regional term, but that's what we call it). It's where moms go to nurse, or comfort a child. It's also where you go when you're in trouble, but we won't talk about that.
Absent a crying room, there will be a library, a coat room, plenty of places for her to quiet the baby if she wants to take the baby. Also, most Kingdom Halls will have chairs in the ladies' room.
Honestly, no one's going to be offended by her walking to the back. If it's anything like my old hall, she'll have plenty of company, lol.
Also, a JW service will mainly just talk about the deceased's life, and the JW's hope of resurrection. I mention this because I've seen a few instances where family were very upset that no mention was made of the deceased's going to heaven.
She'll be fine, no worries.