Anyone else have a toddler who isn’t socialized?

Our daughter turns three the end of this year and I’m freaking out. Next year she’ll be put in pre k, and she is so attached to us I don’t know what to do. She grew up without any kids around her her own age, and when we take her to the park and things she stays with us and if a kid tries to play with her she cries and hides, but likes to watch them at a distance. I realize this is our fault, but due to several circumstances she hasn’t been socialized with other kids (or many other people) as much as she should’ve been, and she has severe separation anxiety when she’s away from us. Otherwise she’s happy, loves to play, explore, I just don’t know what to do. I don’t want to throw her into pre k next year out of the blue when it’s all new, but we live in a neighborhood of all old retired folk and no one in my family has kids her age. We are currently taking her places with lots of kids her age but she is scared of them. Help?