? Depot(s) for children of divorce, aim: control and inspection by only one parent

Hello, my partner is divorced and has two children (born in the 2010s). Relationship with ex-husband is basically okay. We are looking for a way to create portfolios for the children in which ETFs can be saved through a savings plan. Only my partner should have control (also “reading”) (until the children are 18). In the case of a collective deposit, it should be as transparent as possible as to which part belongs to which person. Both children should have the same "opportunities and risks" (i.e