I don't have kids so can't answer from experience but can let you know what i've read:
2) - shipping ahead. There's no address details for Spain that i've found yet. I'd suggest calling
DCL and asking for the address. I guess It may not be possible because the terminal in Spain doesn't belong to DCL whereas it is DCL owned in Port Canaverel. But i'd definately recommend calling DCL to find out for sure.
3) The nursery for very little ones seems to work well for some people. You do have to book the slots, so if you want to get a slot coinciding with an excursion make sure you book it as early as possible. There'll be more info on the nursery on the main board (try the FAQ and search for threads on flounders)