In reply to Col552: Robust- have you thought about going mod surplus my . A quick search reveals that they seem to vary in price but seem about the same as a standard good brand but lower season bags but with kids- im guessing not many will have done that much so its probably better to go on the warm side. They have like a plastic type coating on the bottom that means the bag is super durable.
On the other hand they are fairly large and heavy compared to civi bags.