I've been to plenty of kids' birthday parties that were mostly adults. The kids have fun too, especially if there are activities for them and just general birthday things like cake and decorations. You're not failing her at all. She's not even really at the age when she would play with kids right now (just parallel play) so it's not a big deal at all. If you want more kids, you could always try posting on one of your local groups here or on FB. I've often seen people post on ours to say their kid is x age but they don't know a ton of people in the area and to message if they'd like to attend, and they always have a good response in my area.
She’s only two, you are her bff right now you know? If my two best friends didn’t have kids I’d be in the same boat. I think this is very common for little. Don’t stress it mama.