A possible solution is to use the Downtime function exactly opposite for what it was designed. Downtime was to basically turn off applications with the exception of a select few for critical needs to make the user want to put down the phone and go do something else. In other words, lock down the phone.
In computer security (especially firewalls), we take this approach as well, lock everything down and only enable or allow the bare minimum. So, in this case, make your "operational hours" your "downtime." Create the iOS downtime schedule and map it to when you want your child to only have access to the apps needed for education. This will, by default, block out recreation and social activities, etc. Since in (iOS) downtime, you can specify the contact(s) that can be called and/or messaged, your child won't be without the ability to reach someone important. All of the apps you want to be utilized can be used and none of the ones you don't will have access.
Next, set time limits for the apps you want to be able to be used when (iOS) downtime is off. Since it will be during the "recreational" time, an extra minute here or there won't cause too much consternation.
It can be frustrating when Apple seems to implement something only half baked like to use App schedule based time restrictions, of either having a restriction with at least 1 minute or having no schedule at all. This is definitely something you want to leave Apple Feedback but hopefully this workaround gets you to (or closer to) what you need.