With the paper system, we used to do something very similar to what you’re asking about & as long as they saw the too small child, CMs were always happy to grant the RS without question.
Since they’ve switched to digital RS with the 1 hr time window restriction, I’ve had mixed results getting the RS even though you are correct, theoretically only one of you needs a FP. Rides like Space in DL and RSR in DCA are usually the ones that are the strictest & may not award the RS if not everyone has a FP.
Once nice thing I’ve noticed is that they never check that the name of the pass being scanned remotely matches the name of the person actually entering the queue so you & your wife can switch off using the FP first & riding RS and it doesn’t always have to be you riding first on your FP with your wife getting RS 2nd. If you link your tickets to the Disneyland App in both of your Disney accounts (or just have one of your Disney accounts logged into both phones) then you’ll be able to view all the FPs/RS and use them to scan & the CM won’t bat an eye there.
If you do run into having a hard time obtaining the RS with just 1 FP, note that many rides in DLR that have FP and may give you this issue, also have a single rider line so I would go through with your plan of only purchasing MaxPass for one of you and carry on as you suggest above &go for the Single Rider lines if you need to.
One caveat on Single Rider lines, usually it’s better to use those earlier in the day as afternoon/evenings they can tend to get inundated. Incredicoaster for example will sometimes get a Single Rider line after 4-5pm that is longer than the standby. RSR at night is often longer than the FP line (but not usually longer than standby still).
And as for WOC and F! (and any other FPs you may want to get with your wife & sons tickets to ride rides that he can ride) you can and absolutely should use the free FP kiosks for those. Just note that for WOC and F! both you’ll need to grab them early on in the day but they won’t interfere with you getting any rides since they are disconnected. For the shows, since you would need to get 2 with the physical kiosks anyway, I would grab all 3 at the kiosks so you’re assured the same section/showtime.