Yes, that's the value. Yes, they can share it. Your oldest child just has to be present to pull the funds from the card. Attractions are mostly about $10-12. If you search on here, you can find prices for the specific attractions your ship offers.
I pre-purchased a bottle on the Meraviglia a couple months ago. It was not opened and there wasn't a corkscrew. I took it to the closest bar and asked a bartender to open it.