I think they would be safe. You'd obviously have to keep an eye on them, in the same way you would do (hopefully) at a park or a shopping mall.
The question is, IMO, how *appropriate* a place it is for kids. They will inevitable become bored and will want to run around - the club might be packed, are you going to chase after them all night? If you do, will you be able to enjoy the show? How much will you annoy other patrons? Unless you get a spot on the upstais rail, the kids won't be able to see the stage. But to get rail, you'll need to get there really early - thing you can keep them waiting in one spot for an hour before the show starts? Again, they'll be bored and antsy.
At a minimum, loud clubs aren't good for a child's delicate hearing. I hope you buy them those protective ear-cups that jackhammer and flight ground crews/bagage handlers wear. Please invest in protecting their ears.
Better yet: invest in a babysitter and leave the kids at home.