Even if you could take a camera in, have a bit of consideration for the people around you who have paid a lot of money to enjoy a show. The last thing they would want is to sit next to someone click click clicking throughout the whole performance, checking shots etc...
Even at music concerts, I get so annoyed by people in front who hold up their cameras to record the whole thing on video. Just don't see the point. The quality will be terrible, they'll probably never really watch it again. Just enjoy the show and you'll have the memories.
I believe the Esplanade has a strict 'no photography during performance' policy. And given the fantastic acoustics of the theatre/concert hall, your 'clicks' will not only affect the people beside you but many many people around you. I once watched a show where I heard clicking, only to realize that the culprit was on the ground stalls (I was sitting at the edge of circle 1) when the usher came by to advise the patron accordingly.
I believe you paid good money to watch Cats and so did the rest around you. So just enjoy the show! Don't diminish the experience for yourself and others by clicking away.