I'll be honest. I think intentions were good here. The little girl is still missing and presumed deceased, it's a recovery effort (IMO). People may have felt the Birthday celebration was ghoulish. I don't think it means they don't care. They simply don't know what to do. More professional boots on the ground are most likely needed. All my opinion.
I also think Lynne's intentions are good but she makes some bad decisions. Allowing the part of the event (that we can't discuss) that over 1000 people objected to was a bad decision. That's 1000 people right there that she offended and obviously would not be participating because of it. Secondly, it was on Mother's Day---people undoubtedly had plans for that day so couldn't attend. If the event had been a search that was being conducted a day after she went missing, I'm sure there would have been a huge turn out despite it being Mother's Day but a birthday celebration for a child who's missing and presumed deceased? I can see why people weren't willing to change their plans for that.
If her intention was to draw attention to the fact Serenity is still missing, she did that. The event was reported by several media outlets. If the intention was also to honor/celebrate Serentiy's birthday, she did that too. People showed up, blew bubbles, had cup cakes and did that 'other thing'.
The only other thing I can think of that she could do is organize a search. But with all the searches that have been done with no results, I'm not sure there's any point to that.
The best that anyone can do right now, IMO, is to post flyers to keep her picture out there in case she was taken and is still alive. Birthday parties aren't going to help find her or what happened to her.