Also experience this problem, I am admin and I have setup my daughter with a child account. Have also just rebuilt the machine to move to a new HDD so it is completely clean, up to date W10 1809 (17763.195) with very little else installed outside the default OS (steam and spotify are actually the only things installed).
Obviously the MS family setup isn't working properly - doesn't seem to have any negative impact I can notice apart from the annoying message.
Edit:
Also suggest people check the feedback hub, there are quite a few reports of the same issue and not that many upvotes. MS have said they are looking into it on there but that was three months ago - Feddback hub and upvotes are the best way of getting MS to fix things.