The upgrade table is here:
http://www.virgin-atlantic.com/en/gb/fr ... _table.jspI'm not sure what age the children are but assuming they are occupying their own seat you will need 50,000 miles to upgrade all 4 of you from Economy to PE and a further 50,000 to upgrade from PE to UC. These numbers are each way. If you bought an upgradable Economy class ticket and wanted to put all 4 of you in UC in both directions you would therefore need 200,000 miles (50,000 Economy > PE then 50,000 PE > UC and the same again on the return).
It's worth running the numbers to see if an ex-EU departure might be cheaper.
For 1 Jan 16 and a 10 night stay (1 Jan 17 is too far away to get a quote yet) gives:
LGW - BGI Economy £737, Premium £1429, Upper £2449
DUB - BGI Economy £562, Premium £1458, Upper £1377
Don't go PE from DUB, that's a terrible deal! For economy this probably still doesn't make much sense once you factor in positioning flights, a hotel, food and entertainment. For UC it really pays - for your family you would save £4288 if you were buying cash fares.
Basically you have to decide if an extra ~£600 each to sit in UC is worth the hassle of a trip to Dublin vs the cost of accumulating enough miles to make it happen using M fares from LGW (which will be more than the discount economy £737 quoted) + the extra taxes for the premium cabin.
I hope that makes sense. Of course the whole "upgrade with miles" is a non-starter if you can't find reward availability for 4 on the dates you want.