the weather will normally be clear and sunny by day but still quite chilly at night. the tourist season officially opens on 1st may so all the commercial places will be closed. you may have trouble entertaining two young teenagers - reorts like cala llonga, es cana and the like will be like ghost towns.

eating out in ibiza is a wonderful experience any time of year - though in april you can't go wrong because the only places open are spanish style - this means good food aimed at local people so therefore very reasonably priced.

I don't know how you are planning to travel to Ibiza, however I'm not sure there are any direct UK - Ibiza flights at this time of year.. maybe the odd operator will have started.

Like the last poster said, it will be very quiet, but you may enjoy that. Only local places will be open, definetly not many bars. It gets busy quiet rapidly towards the end of April when the tour operators start up.