Is it permissible to designate Saturday for visiting relatives, knowing that it is a holiday for Jews, and given the children's arguments that there is a holiday on both Saturday and Sunday?

Visiting on Saturday is not a forbidden imitation [of non-Muslims] because it is not a holiday for them; rather, it is easier for visitors. Visiting relatives is prescribed at all times, and dedicating a specific day for it is permissible depending on the circumstances. Visiting on Saturdays is not exclusive to Jews; rather, what is exclusive to them is taking it as a holiday. Ibn Umar (may Allah be pleased with both of them) used to come to Quba every Saturday because he saw the Prophet (peace and blessings be upon him) come to it every Saturday. It is safer to alternate between days if possible, to be further removed from doubt.