No one defensive scheme is good against everything. 5x5 flag football is an offensive game (assuming the coaches know what they're doing). Consequently, a part of coaching defense against good offensive teams has to include taking chances. And by chances I don't mean taking unnecessary risks. I'm talking about stuff like deciding whether to send a rusher into the strength of a formation or the QB's throwing hand (if they're different). Whether to abandon a flat in order to press the middle. Etc. Against a good team, sometimes you just have to guess. Hopefully you guess right more than you do wrong.
Quick throws will neutralize a 10 yd rush and make it basically 4 on 5. However, if we see a lot of quick throws to the outside, occasionally we'll send a rusher off that corner and leak a defender from someplace near the middle/LOS into the flat on that side. The rusher isn't bringing a ton of pressure, but most QB's have tunnel vision when he's in their line of sight. And while they absolutely see the hole he's leaving on his side, they don't always see the defender flying into it from underneath. Pick.
Most good QBs (and good offensive schemes) are 100% about timing -- they read either Rush or no-Rush. We stunt and do what we can to disrupt that, including delaying rushes, especially against teams that like to strike to one side. Occasionally we'll line up two underneath defenders at rush depth (one on each side + a middle linebacker up near the LOS). Mike drops down the middle and the two other defenders read their keys as they leak up into their zones. The QB reads "No rush" and settles, but all were doing is taking a mississippi or two to see which flat the offense is attacking. If they strike quick, we have an extra tackler at depth. If not, our free defender takes off after the QB, who, because he was settled, is now more apt to rush his throw at a point in a receiver's route he's not used to throwing.
It takes some practice, but consider teaching your kids how to banjo against bunch formations (basically zone for 5 yards, then man). A majority of trips formations will do one of two things (i) set up the weak side, or (ii) wash out the strong side only to leak a receiver back to it. A banjo call can be very deceptive if your defense shows man, but your playside corner checks off the split receiver at the last second and stays home. We get a lot of takeaways disguising corner responsibilities, especially against stuff where the QB rolls to his side with a dragging center or some sort of out pattern to that side.
At the end of the day, Belichek is probably right when he says defensive scheme is less important than execution (and the talent you have on the field!). But if your team is anything like ours, your kids play up against good teams and down against bad teams. And against a good QB, they're going to get gashed and make mistakes. Just tell them to (i) make 'em big, and (ii) make 'em going forwards, not backwards.