@SeryjVolk are you sure we're reading the same story? It was specifically stated that the condition was "her child" not "their child". The mother didn't go back on the deal, the father did.
Besides, the father looks like someone who would take another wife anyways even if the mom did love him. He seems like the unfaithful type. Maybe that's why the mom refused to root with him? And what's more, that idiot guy didn't even try to gauge the MC's abilities even knowing he's a child of a Dryad. If he think with his brain instead of his genitals, he would have at least tried to curry the mother-son's favor (considering the mother-son's from a superior kingdom) instead of shunning them. He's unfaithful and an idiot, that's it.