Doidontimmm says, "Is it a room only rate? If so, no problem but please ask him to tell the hotel for emergency purposes. If they charge per person you are being unreasonable!"
Some thought the less the hotel knew, the better, "If it's a great big Hilton type place they probably won't even notice."
Screamanger also agrees that honesty may not be the most practical policy, "I have done this with my [husband] several times. He never mentions me and it's never been a problem."
Kerberos was on the mum's side too, "Personally I would just do it. Chance of an emergency are so slim as to be negligible."
ICanTuckMyBoobsInMyPockets says, "Those talking about emergencies and fires and insurance are absolutely batty. Unless you sign in and out of your hotel like you're in school there's no way the hotel will know or care how many people are in the room. Worked in hotels for many years. Alarm sounds, everyone gets out. Anyone left behind is behind a fireproof door and will raise an alarm themselves if they're trapped, or by their roommate. Fire fighters don't count in and out and don't go looking for additional people."
Thetrollfairy, and many others, were concerned about his work finding out, "If the hotel notice, they'll add the additional charge (if there is one) to the card which booked the room which means it'll get back to his work."
Codenameduchess agreed, "What would the repercussions be if your husband's employer found out? They could quite easily take disciplinary action for him turning a business trip they have paid for into a family holiday."
Meoldbamboo had a cautionary tale, "Someone at my husband's work was sacked for this (one of a few issues) as he used to do this regularly when he and his wife needed to catch a plane for holiday: meetings weirdly coincided with a hotel stay. Their company policy does not allow it. Also the company insurance may not cover you in case of an accident.
Honesty is the best policy I believe (but I'm a bit of a stickler for rules!)."
Others worried for the stowaway baby, "I can't see hotel staff worrying about an extra person in the room. I can see them being worried about a baby staying in a non-babysafe room. Rightly."
Yearinyearout thinks this is a big storm in a teacup, easily solved with human decency, "I've done this before several times both on my own and with a child in tow. The rooms were just booked as a double room rate so it wasn't a problem. He's even been and asked for extra towels for me, and I've picked up an extra key at the desk on arrival, never been a problem. He did tell the hotel I was joining him but they wouldn't bother to notify the company. Just go and enjoy yourself!"