Two more in case you need some more:
https://www.webmd.com/parenting/guide/sleep-children#1
https://www.cbsnews.com/news/new-sleep-guidelines-for-babies...
Many people assume a lot of things that when investigated turn out to be false.
Often assertions don't match their backing: for example you said " The problem with this logic is that the younger the child, the more sleep they need."
However that's not what your first study says. It gives broad ranges, so it concedes that younger children can need less sleep than older children, it's not tied in a simple way to age.
You perhaps meant "on average", which is quite reasonable, and having the source you based you statement on can lead one to assume you were being brief rather than errant.
FWIW those sources look like you quickly googled it; and so weren't really relying on any particular sources.
TL;DR your source supports my observation and contradicts your claim as written.
Make your schedule malleable for your kids and within a few days you'll pretty much know exactly how much sleep they need.
For my kids the trouble is "when" they want this sleep. My oldest goes to bed "early" and rises early. My youngest could stay up until 2-3 AM and sleep until noon (as she did this summer).
You can see both the min and max of all ranges go down with age.
I'm not sure how you could have derived your conclusion based on these results. Are you saying because ranges overlap they aren't valid or shouldn't be used as a guide?
We also don't need to run a scientific study to know that "if the kids are cranky it's likely because they are hungry or thirsty", "it is highly likely that the kid is lying about the fire-breathing goat breaking into the house and breaking the lamp", or "most kids like to eat candy". Again, plenty of interesting science to be done about the reasons for those things, but science does not require us to be that skeptical.
There's a scientific reason for this, too, which is that whatever study you might be interested in conducting on these matters, "the common experience of humanity" has already run a study a billion times larger, and with more statistical power than you could dream of putting into a study. If humanity is wrong about those things, your study is just going to make some of the same errors. It is not productive to run a scientific study with orders of magnitude less statistical power than the data we already all possess.
(And before you go "but what if they're wrong"... I covered that already in the term "statistical power". We are not obligated by science to pretend that nothing not produced in a study has a statistical power of zero. That's a modern perversion of the idea, and causes some serious philosophical bootstrapping problems if you take the idea seriously.)