As someone on the other side, I think it's hard to know what to do for best. My cousin died 5 years ago this year. I thought about sending flowers, but my Auntie, understandably, still finds it incredibly difficult and the last thing I wanted to do was make it harder for her.
Also, last year, another cousin had a little girl who was stillborn. We didn't talk very often before and I haven't actually spoken to her or seen her since. It seemed disingenuous to me to contact her just for that reason, but I was thinking about her and her little girl.
I doubt that they have forgotten - although the date might have crept up on them, if you see what I mean. From my experience, I think it's more likely that they just don't know how to handle contacting you, so they're avoiding it. If you're close to them, you could maybe mention it, not in an accusatory way, just a 'it's okay to talk to me about her/remember her' way.
I hope that helps a little, and wasn't just waffle.
for you and your beautiful girl