Isle of Wight tips - definitely reuse the Robin Hill ticket. They are open until 10pm at the moment with lit up woodland so nice to pop in of an evening.
The paddling pool at Ventnor is a good way to spend an hour or so and the shingle beach there is a novelty after lots of sandy ones.
There's a new ish pub near Lake Morrisons where kids eat free 3-7. Not particularly special food but perfectly edible.
Water play area in Ryde on the sea front is free and the playground on Sandown sea front is a good one.
The Griffin pub in Godshill is worth stopping in for a drink because it has a maze in the garden, a bouncy castle and an area to kick a ball around. Chequers in Niton also has a great garden for kids - and they do an awesome carvery. Fighting Cocks near Amazon World also has play equipment outside.
Hopefully the weather will be nice and you can just spend lots of time on the beach.
Parking charges aren't too bad - if I remember correctly it was £3,40 for 2-4 hours and maybe £4.50 for 4-6.
Google to see which places have carnivals on too. Last week we saw a bit of the Sandown one which was 'interesting'.
Garden centre between Lake and Sandown has fish to look at which can kill a bit of time on a wet day.
If you're sailing from Southampton go early, pop the kids in the ikea crèche then feed them meatballs in the cafe and they'll only need a sandwich for tea when you get there.