I think you need to be careful with this.
They are OnBusiness miles, which means you pay taxes and surcharges, which will be about £250-£300 per return ticket to the Caribbean or US.
This is the same as the normal BA miles, with the exception that availability is MUCH better.
But you still have to pay the £££.
It's generally considered better to upgrade with miles, but you won't have enough to upgrade from WT+ to CW, and you can't upgrade from WT to CW (only one class upgrade).
If it was just the two of you then it would make sense to book flights at peak times, but as you have to pay for two further flights, that's not really such a good idea.
Here's some ideas:
Grenada, in early December
Passenger Fare Taxes, fees and carrier charges per person Total price**
Adult £ 326.00 £ 341.11 £ 667.11
Child £ 244.00 £ 341.11 £585.11
So you pay
£341.11 * 4
compared with £2504.44
which is a saving of around 26%
While this is reasonable it is not exactly free flights.
We went to Grenada 5 years ago and paid £635, total for 2 adults, 1 child and 1 infant. The airline in question went bust shortly afterwards, but not while we were on it thankfully...
Grenada is lovely but expensive, we went to a restaurant there that was previously in London, the owner said he charged much higher prices in Grenada than he had in London because the rents were so much higher.
You can ask here: www.flyertalk.com/forum/british-airways-executive-club-446/
for ideas about how to make good use of your points.
It may be possible for you to sell them.