Whether an organism reproduces sexually or asexually has not real impact in the rate of evolution. Bacteria generally reproduce asexually, and they evolve faster than other organisms.
So long as mutations are occurring, it doesn’t matter much how those mutations are passed onto offspring. Those offspring with beneficial mutations will survive better and ultimately dominate the population. Those with detrimental mutations will die off. The exact same principles of “Darwinian evolution” apply and the work exactly the same way. All that sexual reproduction does is allow for beneficial mutations to mingle with unrelated lines. It doesn’t produce novel genetic material and it doesn’t alter the survival advantage/disadvantage of any genetic trait.
Ask your son to imagine what would happen if one individual in a population of asexually reproducing organism, say a lizard, developed a mutation that allowed it to run 50% faster. Obviously that lizard would produce more eggs than other lizards, and the lizards that hatched from those eggs would produce more eggs and so forth. Very shortly the slower lizards would starve to death because all their food was stolen by the fast lizards. The whole population will evolve into a population of fast lizards, exactly as it would if the lizards had been reproducing sexually.
The only difference is that in the case of the asexually reproducing lizards, the genes form every other lizard alive at the time of the original speed mutation will become extinct. The entire population will be descended from just one individual.
The reason why sexual reproduction has some evolutionary value is that it allows for the conservation of multiple gene lines for multiple survival traits.
For example, in an asexually reproducing population, if the climate gets hotter and drier, then an individual with mutations that allows it to tolerate heat better will produce more offspring. And an individual with mutations that allows it to tolerate dry better will also produce more offspring. But no individual in the new population will be better at tolerating both heat and dry. Instead we will have a mixed population of dry tolerating individuals and heat tolerating individuals. And if heat tolerance provides a bigger advantage than dry tolerance, then the heat tolerance genes may actually become extinct *despite *being advantageous. To produce an individual that tolerates both we need to wait until one of the heat tolerating individuals evolves dry tolerance or vice versa, which may never happen.
However if a sexually reproducing population is faced with a warming and drying climate, then individuals that tolerate heat will mate with organisms that tolerate dry, and the result will be an offspring that tolerates the new climate much better than either parent. In a very few generations the population will be composed solely of heat and dry tolerant organisms.
As you, and you son, can hopefully see, sexual reproduction doesn’t actually increase the pace of evolution of itself. Beneficial genes still need to be produced randomly and be passed on to subsequent generations. All that sexual reproduction does is allow beneficial genes from different organisms to combine in novel ways.
That is not always a benefit either, since it means that if a genetically perfect organism evolves in a sexually reproducing population, it will become extinct in a single generation when that perfect gene combination is blended with less well-adapted genes from the other parent.
As a result, the general rule is that asexual reproduction is an evolutionary advantage where conditions are stable because it allows a gradual but continuous progression towards an optimal form. Sexual reproduction is favoured in unpredictably changing environments because it allows for the rapid combination of multiple novel solutions.
There’s a reason why most asexually reproducing animals live in the water.