I swear that I wrote up a gigantic post about attempting to lock down a chromebook but I can't find all that stuff I wrote. Maybe it was in text only and I never uploaded it. Bottom line, there are only two things you can do. First of all, there is only one single parental control product for chromebook, Mobicip. Mobicip has a lot of good features; it monitors websurfing and makes reports, and has some remote locking capability. But oddly access to the app store is completely unrestricted and can't be turned off. The other option, which I had to go with is to install FamilyLink, which did everything I wanted but is such a beast to install and has some things which are too tightly controlled (only YouTubeKids allowed for example).
Mobicip: https://www.mobicip.com/