Well if it’s for the children pfsense will do all you want ! Just inspect it in a VM, it has a schedule feature you can define, then call that schedule in your rules.
You can apply that schedule to the whole vlan easily.
With Whatsapp that’s a bit tricky … I manually add all the allowed ports to an alias list including Whatsapp (all other ports are then obviously blocked), so you could go that route but it might be more effort than you are prepared to invest. Then combine your whatsapp port with a schedule.
I’d say doing things by device or IP address will basically be more effort and prone to errors but the vlan will give more precision but might be more effort depending on your knowledge levels.
You can of course punish your children immediately by disabling a vlan rather than by waiting until 22:00 
Oh just seen that Circle 300$ for life, wow big money for old rope !