I don't think there are readymade statblocks in 4e for tentacles either. I might be wrong. But in 5e the creature resilience/offence ratio is not fixed. There are some creatures that are more glass cannony, some that are more tanky. So having hard hitting but not so resilient enemy is definitely a thing that is perfectly within the spirit of 5e monster design. Though again, having or not having such enemies has absolutely nothing to do with player agency, so I have no clue why you're even talking about it.
Yes, these are things in 4. What they have to do with player agency? Anyone can list random things from a game.
And that se has some dodge power? Why would you know that? And whilst it is indeed reasonable to assume that little girls are not resilient, technically the characters would just know that she looks like one.
Why would you know his capabilities? Why would you know his disposition? Why would you even know he's the girl's dad? Maybe he's a priest of Cthulhu, who has lured the girl there on false pretences to sacrifice her to the tentacle monster? You don't know!
Let's assume for a moment that the GM would want to create situation like this, except that there actually is some surprise. The girl actually is a powerful chosen of the Elder Gods, controlling the tentacle monster, and 'father' likewise is her mind controlled puppet. This is actually a trap to lure the characters in. How would you present that then, if the usual practice is to list everything openly, whether the charters could actually know it or not? In fiction everything looks the same, yet now you cannot provide the same mechanical information (or if you do, it is a lie.) This clearly demonstrates that there is massive disconnect between the mechanical information and what the characters would actually know.
I mean if the GM was actually so skilled illusionist and railroader that I didn't at any moment feel railroaded then it indeed would be perfectly fine. Now in practice it is extremely unlikely that a GM could do this, and the easiest way to not have the players feel that they're railroaded is to not railroad them! But yeah, how it appears is what matters, not how it was actually done.