Just ignore!

I think you need to find the backstory first. Perhaps she had already sent out party invitations for the kids party when she found out when the shower was?
Will you see her today for Easter? Maybe she thought that so that helped her justify it?

Could it be that the adults that were throwing your party made a judgement call on who to invite? Honestly, it may have been a oversight on their part, not realizing that you would bring the dresses expecting the girls to be there.

Back in the B.C. era I was a flower girl a few times and I never went to anyone's shower, even if my dress was made. Oh, and yes, once was for my uncle's wedding in Canada.

Try to give them the benefit of the doubt and find out the story.