[ZenGum]

What a dilemma!
If you can get a team to click immediately, I believe England are looking for a new coach.

Seriously,I think it's great that you are worrying about the other team's morale, but I think you're overdoing it. Remember, they only get beaten by your kids once, then next week play someone else, and maybe win. One thrashing a season is acceptable. You aren't really responsible for the other team's feelings, except for keeping the behavior of your kids polite and sporting in victory. No gloating.
I think you'd do more harm to your kids (and they're your primary responsibility here) by stopping them from doing their best. I think you're already doing enough to make things harder for them with your creative substitutions, and your thinning of your team.
The only risk I see is your kids turning into arrogant jerks because they always thrash their opponents. How to behave in victory is as important a lesson as in defeat. Apart from that ... let 'em win.