We are very regular visitors and have never been to either, actually we have never been to any of the pay for attractions - we good about 5 times a year. Personally the Islands beaches are so beautiful and varied that even on a cold windy slightly rainy Feb day you can find things to do on the beach. We also love walking at Firestone copse and Culver Down.
Flamingo park was fantastic for our then 3 yo and we were there for hours. Amazon World we were in round and out in 5 minutes and it was really tiny. Didn't help that there were 2 school parties bundling round behind us- total waste of money.
Robin Hill is another good place to go, and you can go back again for free the same week once you've paid once. Blackgang Chine is a bit naff but little ones like it.
The 'Isle of Wight Zoo' is next to Dinosaur World, a bit of a walk from Sandown.
I'm told DW isn't great unless your kids are the right age to be obsessed with dino digging, but the zoo was good - just that it has two zones, Lemur+Monkey World, and Big Cat World. And almost no other animals, so be warned if your dc are looking forward to seeing a giraffe/meerkat/crocodile. Also go at the start of your holiday as your ticket is valid for a week (we went on the last day but they helpfully refused to charge for dn - 3 and up are supposed to pay) There's a good playground on the way back to Sandown too.
Suspect the Amazon World is a more diverse zoo.
The steam train was a great hit with ds and dn (2 and 3), and then we got the bus to Butterfly World where the butterflies were OK but the FOuntain World in the back transfixed them for ages! Were planning to walk there but the road was way too dangerous for a pushchair! Remarkably nice bus drivers on IoW - they refused to charge us to or most of us from Butterfly World and wouldn't take money for all of us when it was raining, either!
Botanic Gardens in Ventnor was great for 2 and nearly 2yo last year but a 5yo might be a bit bored though there's a good playground. The super-crazy golf in Shanklin or Sandown pier might be a great hit for your ages.
ooh, thanks everyone. we are going soon and i was wondering whether to buy tesco vouchers for amazon world, but it does look small even on its own website! i quite fancy going to the botanic gardens in ventnor so i think we will do a trip there for the day. thanks for the tip re flamingo park perhaps we will go there although it is a long way from freshwater! we are def going to the train though - train mad DS! thanks again...