Solved (for me). I was struggling to add my youngest (4yo) several times without success (Windows computer - Firefox browser) Then I abandoned that and tried to add my oldest (9yo). She has a Fire tablet (modified android OS) and Silk browser. Absolutely no problem to add her to the household account. I tried again for my youngest and success from the first time. Just filled in the form and at the end you get a message that they don’t comply with the age for executive membership but can be added to the household account. Click in the confirmation mail and ready.
Haven't tried other operating systems or browsers so I don't know which one is the culprit.
Good luck