I'm sure if private owners rent out their cars to the fleet they can also subscribe to services that for a flat fee cleans it when necessary, and after last user before it's sent back to you. There would probably be the choice of what charge level it should be at before sending it back as well.
As for kiddie seats, dog cages etc every car would just be rated for what it has and even if only 1/20 cars lets you bring your animal that should be more than enough.
What number of households actually have kids 0-10 years old anyway? 20%
There's been some speculation about FSD prices going to be $200k or something since it'll be so valuable for renting. I'm sure if the production is big enough to actually have cars available to sell to non-renters there will be a FSD version not allowed to join the fleet that cost something similar to now. So commercial FSD $200k, personal FSD $7k
Actually I'm pretty sure soon after FSD is released you will only be able to subscribe to FSD anyway. Again, very different prices for commercial/private.
All of those things will be quickly figured out once FSD actually works. So we'll be debating this for awhile