Reiffer's right. Sounds like VS have dropped the ball here, and as spiceke mentioned as you would have been on the same PNR, there's no way they should make a mistake like assuming you were a lone traveller. The compensation seems derisory, and you should tell them as much. If it doesn't even cover the cost of upgrading, let alone compensate you for splitting up the family between cabins, they really should be doing better.
Re spiceke's other comment regarding seating families together, I'm pretty sure that's one of those 'best efforts' conditions, since sometimes you simply can't find seats together on a full plane with lots of other families (and if you're one of the last to check in).
Re spiceke's other comment regarding seating families together, I'm pretty sure that's one of those 'best efforts' conditions, since sometimes you simply can't find seats together on a full plane with lots of other families (and if you're one of the last to check in).