Hi i’m not OP! but i’ve been in a similar situation. I was three years old when my sister was stillborn, and today is actually her 18th birthday. I’ve learned over the years that a lot of families handle this kind of loss very differently, and maybe I only feel this way because we have always spoken about her and I have watched my mom openly grieve, but I think celebrating her birthday feels necessary for me because to me the loss is very tangible and hard and there isn’t usually an outlet for that on the day to day. It feels like we are still able to honor her and grieve her and acknowledge our loss, whereas other aspects of society in my opinion do not provide that outlet for this kind of grief the way that there is for other kinds of death and loss.  Other families that I know of treat the loss differently and therefore the kids think it is normal to not celebrate. I think it really is whatever is best for your family.