They must be kidding...they went to a stage play after having purchased two tickets, and just expected that they would be permitted to bring two small children, one of which is not a baby but five years old, and bring the kids in to sit on their laps without being charged for them?
Many theaters and sporting events require tickets for everyone regardless of age. They may have been doing this couple a favor by offering to let the baby in free.
I would think that unless it was a play for children, they shouldn't be bringing such young children to a play at all. Most plays I have been to have an age limit. I remember someone trying to buy a ticket at the TKTS for a Broadway show for a party that included a three year old and she was told that the only theaters available that would allow the child in would be the ones showing the Disney musicals. And if they went to one of those, they would have to purchase a ticket for the three year old. If it's community theater or whatever, then they usually sell to families of all ages, but a professional theater company is more likely to not allow babies or very young children.
I think besides the reasons for restricting age at plays that have already been mentioned, there is the fact that in a stage production the performers themselves need to concentrate on the show and not on babies acting up in the audience and parents getting up to take them out.