Sadly, not much you can do then. I'm fairly sure if Mum cat is as ignored as it sounds, she'll hide them well to protect them rather than show someone where they are.
But get on top of the spaying voucher; at least if you do that you can stop it happening again. I remember this happening years ago with a couple who had a cat that looked like a female version of my boy Toby - I'd let her in my house when I was half asleep not realising it wasn't Toby because he was already in!
I didn't know they'd not had her spayed (I just assumed everyone got the op done, rather naively) and was told one day that she'd got pregnant, had her kittens and then disappeared. I was horrified and I think it's one of the things that's led me to be as interfering as I am with regards to cats.