I noticed in the credits that there is a "Set Teacher." I'm wondering if that is a teacher who is tutoring Will during the filming days? (I can't imagine what else that title would mean.) I agree with another poster who said they probably have assistance during outings - they didn't show the nanny on the San Francisco trip but that definitely would have been a big help to have her along.
Another difference between Will and Zoey, beside just their chronological ages, is that Will is more verbal than Zoey. He's starting to express himself in short sentences, while Zoey doesn't seem to speak yet. So they are able to set down rules and can expect Will to understand and follow, whereas with her, they can't be sure how much she's processing.
I think with a combination of a nanny and a teacher during filming days, they are probably on the right track with safety and discipline.