We didn't go P&O so no specific advice there, but we did take our 18mo on a short cruise and it was great. She spent most of the cruise in the pool - there was nothing about needing to be toilet trained (she did wear a swim nappy). In fact, there was a wading pool specifically for babies and young toddlers.
There was a great variety of food so no problems there and it was very easy because we didn't have to lug gear in and out of everywhere - we just had to make it to the docks and check it in, then the most we had to carry from there was a day pack.
The cruise line allowed children from 6 months old. There were a number of infants on board.
I'm not sure that their babysitting service caters for younger children, though (actually, I think the in-room service was from 12 months old? we didn't use it so can't remember). We thought about putting our older one into the kid's club but had such a good time hanging out together as a family that we didn't bother in the end. So can't tell you much about that but I don't think it's necessary, depending on what you're wanting out of your holiday, I guess. It may be less of an issue if you have your parents along to share the childcare with.
Presumably over seven days you would have some stops and shore excursions to break things up a bit. We went for three days and it was a bit too short. Five days would have been just nice.
We actually decided after that (it was our first cruise) we should look into doing a family holiday with the grandparents etc. It can be quite hard to cater to everyone in our extended family at once as people have such different styles of travel, from the backpacking-mountain-climbing-extreme-sports-travel-geek-cultural-immersion end to the please-just-give-me-a-martini-a-book-and-a-sunlounger end. Cruise companies are pretty savvy at catering to a wide range of tastes and age groups, we found out.
But sorry I can't give you specific P&O advice
This post has been edited by beabea: 13/01/2013, 01:24 AM