Since my original post I installed 10.5.1 and upgraded to 10.5.3. I discovered the shortest possible setting for time limited computer access is 30 minutes (per day).
I need a range where the low end is 10 minutes.
Sigh. Apple engineers need more children.
I also don't recommend regular people install an OS X update to an existing system until the DVDs actually ship with 10.5.3 or later. My install experience was unsettling.
Both the Nintendo Wii and computer access (Flash based games) have been big behavior modifiers for the #1 son.
In particular, we used Wii/computer time rewards to get him dressed and downstairs in the morning. His normal tendency was to wake up his sibs, which they did not appreciate.
Combat on first awakening is a bit draining.
Problem is, he's having increasingly severe problems stopping at his allotted time. We had some success with various incentives and disincentives, but he's broken through them.
So he's off computer and Wii altogether while we reevaluate.
Which is why, despite knowing that it's still pretty buggy, I placed an order for Mac OS X Version 10.5.1 Leopard. The killer feature is that 10.5 allows parents (administrators) to set time quotas on user accounts. When the quota is exhausted, the account logs out.
I'm going to give it a try, in the context of limiting access to less enchanting web sites. If he can manage that we'll walk up the enchantment scale. Stopping is a key self-regulatory behavior, so we'd like to use this to teach him some self-monitoring skills.
It won't be easy. It's never been easy. (Ok, so baseball is surprisingly easy ...)
PS. The current version of OS X is 10.5.2, the update from the DVD version is free. We expect 10.5.3 to be released in the next week. My guess is that 10.5.4 will be quite high quality and 10.5.3 will be respectable.