Tbh I think you've made things tricky if staff know, you're visibly pregnant but you've chosen not to tell your class. If it's seen as common knowledge then it's not reasonable for you to expect people to know you weren't telling your class unless you've explicitly told them that's the case or to expect them to act like they don't know.
Also, you don't know if she announced it to the children (which would be poor form as that would assume she knew they didn't know) or, more likely, that it just came up as part of a random conversation.
It's possible other children already knew, or their parents strongly suspected, then when asked "Ms Dinner lady isn't Ms teacher pregnant?" coupled with it being common knowledge amongst staff the dinner lady either didn't see a problem with confirming it, didn't think quickly enough, thought it was daft to deny it.
It's OK to feel miffed if it wasn't your plan but I think you were setting yourself up to fail anyway.
I didn't tell my class for a while, some knew but kept it to themselves, I wouldnt have been upset though if they found out.