I had this problem too, but eventually it registered as complete long before any of the children reached the top of an adult career. At the point it completed, two children had topped a teen career, and one of them had aged up and started an adult career (at level 3 due to having an A), and a third sibling was at level two of a teen career. I'm not sure why it then triggered - it may have been due to starting an adult career, or the total number of career levels between all the kids passed ten (3 + 3 + 2 for teen careers and +3 for adult career). Or it could have been just a delay in registering, or some other glitch. I've noticed some of the aspirations that involve tallying something don't always register correctly until I've either traveled or exited the game.