Chistening invitation

Dr._Shoe

Some people are just professional gatecrashers.

I would have said, "Oh no, there's no need for you to go to such trouble, taking the day off work or anything," and then not mention it again, much less when it is.

TBagpuss

I would simply not mention it again. If she asks you then you can simply say "oh, my daughter is organizing everything, I think she is only planning to invite close family"
Which is true.
If your colleague attends the same church as your daughter then she will know about the christening as it's likely to be mentioned in other services and the parish magazine, and she will be free to attend the service which , as others have mentioned, is public.