Do the parents (and your clients now) live in a different city?  If so, I would claim it as Unforeseen Circumstances.

If the parents are within reasonable driving distance of their old house, something seems amiss and I can't see it.  Clients were probably already commuting to work and driving the children to daycare, so driving to the parents (essentially a daycare) wouldn't really be a need to sell the house.

But Unforeseen Circumstances is a facts-and-circumstances thing.  So it just depends on those details.