Just got this router a couple of days ago and struggling with it too. I've found out you can do this quite easily. Go into Advanced | access control and click add policy. add a name for the policy (eg kids pc), add a schedule (leave it as always) then in the next box select the computer from the list in the right hand box and click OK. Hit next, click block all access and hit save.
In your policy table you should now see kids pc with a tick by the left. That pc now has no internet access. By ticking the box you then block the pc, and un ticking it you allow it access. You can combine this with a schedule to do it at certain times. I'd reccomend the tutorial 50 in the d link tv website to show you how.