The "toddler gets to utilize their FP" argument has never held much for me, because it's not about the toddler missing out as much as it is about the parent missing out on something they want to do do, and in my past life with toddlers that was like 90 percent of life, period.
My personal belief is if you don't have a FP for a ride, you shouldn't get FP benefits for a ride. Under your scenario, the big kid gets the benefit of 2 FPs, and at least one other kid gets the benefit of 2 FPs. 10, maybe 11 for the "cost" of 8, presumably as a reward of sorts for going on the ride with the toddler. I don't see it as a detriment to go on a ride with one or two of the younger kids -- I just see it as a trade off and part of being a parent. That's the way we did it -- heck, one of my 11-year-olds doesn't like roller coasters, so that's how we still do it -- one of us takes two on the coaster, the other hangs out with the one who doesn't want to ride and does something they want to do. I might miss out on two minutes of riding, but I don't see that as a "rewardable" situation, or that I've really "missed out" on anything. In your scenario, the big kid who didn't do Space got to experience Pooh with two kids -- not a thrill per se but still an experience.
I know others disagree, but to me this whole argument is much more about parents than kids, and if you break it down into those typical choices parents have to make between kids -- the softball game versus the soccer practice versus the theater rehearsal -- it doesn't seem like anything different than par for the course.