I'm quite late to this, but for the future...
DS has celiac. When he's invited to parties, I ask what they are going to serve and then I bring a similar gluten free option for him. As he's grown up with this being his life, he's used to it and not only doesn't get upset about it, he totally advocates for himself at parties (like the one time a dad tried to serve DS cake).
As your DD gets older, it's going to become hard if not impossible to NOT send her to parties. Because SHE'LL be upset to not be able to go. You all need to work together to find a system that works.
And to date- when I've reached out to the parents asking what they're serving, I briefly tell them why and I tell them that I just want to know so that I can bring something for DS. This has yet to be an issue.