We used in in-cabin listen service that you activate on the phone in your room,it offer a 2-way listening service, you dial your cabin and you are able to listen, phones are all round the ship, we would phone the cabin every,10-15mins.On the oceana, p&o,also offers a night nursey that you can book your kids into after 10pm-2am,fully staffed.
Yes www.travellingwithchildren.co.uk sell a great monitor that will work anywhere, any country and any distanc.
The mother-in-law does have her uses.
Ocean Village have a great night nursery and also operate a listening service in the cabin where you can listen to your cabin via the telephone. Hope this helps.