Welcome to the concept of “Parents of Autocrossers”!
First off…. it has been said many times: If you can get either or both of your children into a kart owned by someone else (share a ride) nearby, that should at least give them a chance to see if they like it. It is an unfortunate circumstance when youth karts are bought, and not used by the intended driver.
You are correct regarding the classing for your son and daughter. I have to note that even if they are very close in both size and weight: Using a single kart for both classes is (although possible) extremely difficult. With the Briggs World Formula engine, the only required change is installation of a throttle slide restrictor for the FJB driver. Unfortunately, that means a compromise in performance for both competitors, and a hassle for event organizers and other participants, as the two would almost certainly have to run in different heats.
Thank you so much for the response. We are trying to start a kids co-op as we have lots of interested parents. So, getting 2 of them probably isn’t an issue. Thank you for the advice. If you have any more nuggets, I’d happily listen!