I'm guessing she hid the baby on the way in. I don't ever get patted down on the way into a nightclub. No idea why, but it's never happened. DP tends to be; I have some friends that are and some that aren't. It's completely predictable.
I'd never try and take a baby into a nightclub but if she smuggled him in and was asked to leave when the baby became noticeable, as Twitter is claiming, I don't think the nightclub is culpable.