The kids and their careers as they matured
This film's cast of children all encountered difficult times in later years, despite being quite prolific during their younger days.
Mimi Gibson (Elizabeth) and Charles Herbert (Robert) both saw their careers pretty much over within 10 years after Houseboat, IMDB lists 1968 as the last acting credit for both of them.
Paul Peterson didn't really fare much better, if anything his was a harder road. He was a full on teen idol during his years on The Donna Reed Show, but after its end he never seemed to be able to recreate that success. It had to be extremely difficult to deal with that kind of rejection after being such a huge hit. He eventually left show business (as a primary career) and earned a couple of degrees, and became a published author. His biggest contribution is the creation of A Minor Consideration in 1990, a support group for child actors.
A happy ending in the film, not so much in reality.