I think that alot of kids would find it scary to see thier Mum in pain and moaning and groaning. Also there is the possibilty that they could feel a bit hurt and ignored a bit hurt if you withdraw during labour. IMO if you want your kids there you would need to spend quite a bit of time beforehand educating them about what they will see. You might want to consider having a a babysitter close by (in the waiting room) so that they can opt out any time they choose or if any complications arise they can leave.
Also in a long labour they might get bored so maybe they could wait until you actually start pushing before coming in so that they can see the birth but miss most of the actual labour.
In my hospital siblings were allowed to be present but you had to have an extra support person to look after them.