What is the relationship between my children and my co-wife's children?

They are paternal half-siblings — mahram to one another and they inherit from each other under faraid. This relationship is a blood tie that cannot be severed by disagreements between their mothers. Encourage the children to know and respect their paternal half-siblings, as suits their age and circumstances. The father must be fair in gifts to all the children; the Prophet ﷺ refused to witness a gift given to only one child. In inheritance, the children of all the wives inherit at the same rates according to gender. See children & polygyny and faraid.