To be honest, from experience, not a lot. Can't comment on the children's stuff but can't remember there being much at all on our 2 Easter cruises.
There has been a Sunday morning service in the theatre led by the Captain with a few traditional hymns. One was 'Those in peril on the sea' ------ have to smile, it made us chuckle !
Nothing really special - I suppose in the UK Christmas is seen as the main festival, not Easter.
Easter in the UK is, for most, just Easter eggs, Easter bunnies and .... errrr ... a nice weekend break (and fingers crossed that the weather will be good).
P&O just follow this format in that Easter isn't that big a thing for them ----
If that makes sense.