As you've noted before, all sciences have problems remaining to be solved. Any science that has nothing left to clear up is dead. It becomes mere technology.
But fortunately, the universe in which we live is at once remarkably understandable, and hopelessly complex.
BTW, I like to toss in some problems in evolution for my kids. Unfortunately, most of the problems are higher-level ones, that aren't very accessible to them. One I like to give is "why are there no marsupial whales or seals?
Another one (but it's not really a problem because I know the answer) is "why do we see analogous structures on whales, fish, icthyosaurs, and mososaurs?