My children can add each other as friends but I can only add one of them

I have two kids, 13 and 11, who have duolingo accounts. They can both follow each other, and I can follow the 13 year old, but I can’t follow the 11 year old nor can she follow me. We can see each others’ profile pages via direct invite links and from the super duolingo family plan page, but neither of us see a follow button for each other. Is there any way to fix that?