Has anyone gone on a cruise with young kids?

We are considering going on a South Pacific cruise with P&O this July with our girls who will be 4 (almost 5) yrs and 2.5 yrs.

Any info regarding cruising, P&O, etc would be good. We've been to Vanuatu and Fiji so don't really need island specific info, more interested in the cruise ships and cruising with kids.

Cheers!