I assume the league rule is >35 pitches=1 day of required rest. 34 or fewer pitches on Thursday preserves the pitcher for Saturday. Probably not the best plan to pitch an 8-11 year old with one day of rest, regardless of the number of pitches. Also, 7-8 arms is a luxury on an 8-11 team. Usually you are looking at 3 arms--one of which is a bit shaky.
My questions not answered by your post would be: What is the maximum pitches and/or innings allowed per outing or per week in this league? Also, are these games against a 2hr clock or do you complete the full six innings with regularity?
Without knowing those answers, I'd try to use two starters every game day and fill in with your developmental pitchers. Able and Baker start on Thursdays while Charles and David pitch Saturdays. If you want to do more matching up pitchers and opponents, you certainly can because you have 4 days off between cycles. I would avoid using all 4 starters for an inning each on Thursday and Saturday. They will be warming up for two games, pitching innings, and tossing between innings. They will throw more total pitches than if you exhausted their pitch count on a single day.
Relievers Ricky, Ronnie, Robbie, and Randy pitch as needed to the tune of 1 inning each. Don't let them throw more than 25 pitches in an inning and don't start them for second inning if you plan to use the same reliever again on Saturday.
The key will be remaining flexible. For example Able gets the ball to start on Thursday with Ricky the first reliever planned for the 3rd or 4th inning in a perfect world. If Ricky does well and goes 2 innings, you can close with Baker. If Ricky struggles or goes long on pitches, you bring in Ronnie then close in the 5th or 6th with Baker.
If you games rarely go 6 but generally go 4-5 innings, you may want to run Baker after Able then use the relievers for the "if necessary" innings.
Another key factor is how many catchers do you have? Will you need to have any of your "pitchers" behind the dish? I'd limit the number of pitches on a Thursday to under 25 if the kid is going to catch any innings for you on a Saturday.
Really, with so many arms, you have unlimited options. I bet that once you start practicing, the number of arms you really have ends up being less than what you think right now.