Hi, I would get on the web sites and get the brochures(mail easer than going into a TA), some lines do DVD's of their offerings so get those as well. read up what they say they offer for cruise lines they are usualy fairly honest unlike the hotel holidays which seem to over sell.
Read as many reviews as you can but beware of the moaners, you are looking for the details that are important to you like the kids clubs.
All ships have the quiet spots,
P&O will tend to more formal, Princess.RCI, you can get buy with no formal at all but smart/jacket is kind of prefered, NCL is what you want(withn reason).
Allthough we don't have kids(so this is speculation/observation) it does seem that the young ones have great clubs/activities, there seems to be a bit if a gap for young adults but if there is enough similar aged or happy with more adult things then she should be fine. One port on a 14 cruise might not be enough to keep her on-side.(shoe shopping in Cadiz is good).
As for Monaco this is easly done from all the riviera ports, Nice,cannes etc so don't rule out a cruise just because it does not stop at Monaco.
I know it is a long way off but depending on where you live you might want to look at how you get to the ports, UK booking will come with parking/coach/cheap air, if traveling a distance and want to drive considering a stop overnight so you break drive and still get to the port in sensible time, early boarding will be around 11:00am so you can get lunch on the ship and spend the rest of the day exploring, plenty of cheap places withing 2-3hour drive rather than do it in one stretch getting up early.
Definately look at booking through the US, NCL should work out a lot cheaper, RCI/Princess/P&O not such big differences and not so easy but if the saving are there is may ne worth it.
We are currently NCL fans so this is their offering for next year.nothing for May though.
http://www.ncl.com/nclweb/booking/cruisedetails.html?packageId=1637278
Have a look at the AB grade cabin with two berdooms/bathrooms not cheap though with 4 sharing it does bring the price down per person.
Princess Grand class also have a family cabin to sleep 4 with extra bathroom.
An idea of budget? would help give people an Idea what to look for, remember that you will need spends money, tips,drinks,trips and this can add up. for a family of 4 £1.5k min, on top of the cruise fare and probably more if you do a lot of things and like a drink, even DIY trips soon add up when there are 4 of you.(tips will be around $10pppd so $560).
Summary of the main lines/ships doing your desired trip May-July 2009.
P&O Ventura,oceana,oriana
Princess Grand
RCI Independence
NCL Jade
Cunard Victoria, QM2