MegNason

I did it once in 2013 on Jewel of the Seas. This was before nursery's were available on most ships.
The service worked well and we were happy, but there was no way to really reserve things in advance. Basically, Guest Services would attempt to find a crew member who had spare time to babysit.
My advice is to use the nursery. Not only is it cheaper, but they have cribs and the staff is trained quite well.

We went on our first cruise last year with our our two year old.  We asked the genie to arrange in room sitting each evening and he was adamant that we should use the nursery.  I was very hesitant but decided to give it a try.  Wow-was I impressed!!!  I dropped him off before our dinner with his pj's.  When we returned, they had played with him, fed him a "late night snack", changed him, rocked him and then put him in our stroller to sleep.  When we returned, I wheeled him to our room and transferred to the pack n play.  It was the easiest night out ever!!!
After that, we used almost every night and he happily went in.  The staff become familiar to him.  They were incredible and much cheaper than a sitter at home.  They even put him in his sleep sack!!
I would highly recommend a Maclaren stroller.  They are small and very comfortable.  We got the one that reclines so it was great for getting him back to the room.  Good luck!!

@KWofPerth great, thanks!

@DocLC @klaconqueso @RBRSKI Thank you all!

@coneyraven Thank you! Appreciate your help!  My husband loves a good Bloody MaryLand...he will appreciate this!

@coneyraven Thank you!  Yes, we loved Andre when we had him the last time on Grandeur!  Hope to see him back too!!  We had the Ricky + Andre combo and it was the best!