Children are poor men's riches. - i think in Russia this is understood as follows: Poor people have nothing to rely on but their children (in material sense), hence extended families in the past (and hence children become 'riches' or poor man).

На что и клад, когда дети идут в лад? - (i suggest that question mark should be at the end, as it is a RHETORICAL question) Modern language translation: There is no need to have a treasure (i.e. material riches) if your children are very well and successful.
So the meaning is that wellness of one's progeny is valued more than material riches.