The wave hitting the beach is a random event, like the ones discussed here: you will need your villagers to clear it again. The more building skill they have, the better they should be at it -- but I don't know if there's any way to know if they are clearing it "faster" or not.

(As for unfair, well, that's the nature of random events -- some of them are fair, some of them aren't )

For your child with building skill, make sure "building" is his/her preferred skill. Until he/she is a "trainee builder", you'll need to keep putting him/her back on the beach. You need more than just a little skill to become a trainee, so it may take a few tries -- and even trainee/adept builders sometimes just can't figure out the job...but they'll usually try again.