Is the plot of land that the father bought with money he earned during his previous work on roads (before he left it, believing the additional money he was earning from it to be unlawful) considered unlawful? And if so, what should he do? And does its unlawfulness apply only to him, or does it extend to his wife and children as well?

Your father was correct in his ruling regarding the prohibition of the money he took from the truck drivers, because it falls under the category of "gifts to workers are treachery (ghulul)," even if the drivers consented. The gift here is due to his position. He must repent and estimate the unlawful money, returning it to its owners if possible; otherwise, he should give it away as charity on their behalf. As for using the money to buy land, this does not make the land unlawful for him, his wife, or his children.