Web sites access limit by time of the day

Hi,

Could you please let me know if I can automatically restrict access to certain chat web sites by the time of the day on networked home computers on Verizon DSL running Windows XP SP2 and latest Zonealarm Pro firewall? Can I also limit instant messenger start by time of the day and day of the week? I’m trying to restrict kids’ chatting before homework is completed, but still let them browse other sites meanwhile. I thank everybody for any ideas/input.