Access Control is on to limit access but limits me during random hours when it shouldn't

Access Control is enabled on my Airport router to limit internet access for my kids and anybody visit the house. Wireless is configured to be available from 6am-9:30pm. It has worked great but has recently started to limit access to everybody during random hours. (even in the middle of the day when it should work for any and everybody). This includes limiting computers that have been given 24 hour access to the network. If I restart the system, it will correct itself and work for a while. Sometimes it works for 15 mins, maybe a day, and then it just starts to limit access for everybody again regardless of the configuration completed to allow access for certain MAC addresses. Do you have any ideas what could be causing this? It is a huge hassle to have to reboot the router multiple times a day. I can remove the access control to fix this, but that frees up my kids to have internet access all night which is not desirable.