Your request is somewhat broader, but as you mention your kids:
DNS blocks can play a (minor) part in parental control strategies, but they do not exactly lend themselves to time-based controls, and they won't obsolete other network-wide and on-device measures - and certainly neither non-technical, more important educational measures.
Specifically, by-passing your blocks is as easy as changing the DNS server on the device (or switching to mobile data if its a smartphone).
Kids tend to find out how to do that sooner or later.
Even if Pi-hole is in place as DNS server, a client may cache sucessfully resolved DNS records until that record's TTL expires.
Your block then won't be effective until it does and a client requests DNS again.
TTLs vary by domain individually and may be minutes, hours, days,...
And even if a TTL is as short as five minutes, it doesn't mean a client will request DNS resolution again once the record expires. All that happens is that the DNS record will be removed from your device's cache.
It's the software on a client that decides when it requests a domain to be resolved, e.g. if a game would only use a domain once to load a complete level, it may have no reason to request DNS resolution again until another level is needed.
Meanwhile, your kids may happily play through that level for hours.
You'll have a somewhat hard time exercising time-based access control by DNS means only.