On kid tiering:
The farthest I'm willing to go before things get really stupid is to designate tiers for them. For example:
Kid Tier List:
A:
Morgan
Any Avatar's Child
B:
Yarne
Kjelle
Cynthia
C:
Owain
Gerome
Severa
D:
Nah
Noire
Brady
Inigo
This list has the stipulation that "if the kids were in the actual tier list, they'd probably be somewhere in there or below, with exact placement varying based on parents and parents' stats".
Okay, so let's hit Henry vs. Tharja now.
The main thing that I want to hit here is supports. Namely, Tharja doesn't have great ones. Her best supports are either Libra (patching up weak points, like her mediocre Defense and godawful Luck) or one of the few very sought-after speed supports (Gaius, Lon'qu, and Gregor), which do nothing for her mediocre Defense or her damage dealing. Anna is a dead end support that does nothing other than delay a more worthwhile support, so I don't consider her a viable option for any long-term use, as she actually weakens Tharja in the long run by watering down her supports.
Henry, on the other hand, has two potential supports in Dark Flier Sumia and Cordelia, both of which provide him with exactly what he needs to get ahead- Speed, Resistance, and Magic. His Defense, Skill, and Luck are enough for him to do the job, so having a support that targets the exact stats he needs is a godsend. As for it being "greedy" it's no less greedy than taking one of the limited speed supports in the game, especially when you consider that the time Henry shows up is about the time that Frederick is either being dropped or pairing up with Cynthia (depending on his amount of EXP gain and whether or not you have early access to Cynthia). Considering what Henry gets out of the support, it's hard to say that Henry would ever be seriously used without a Dark Flier support. Also, by now Sumia's strength is probably starting to taper off pretty hard (6+45% Strength does not a murder machine make) and we're in Valm, land of hard bodies, so it's not unreasonable to say that she's moving on to greener pastures.
Furthermore, it's not unreasonable to say that when both are doubling, Henry is winning handily. His superior Defense, Luck, and Skill make him much more reliable, and at base it takes about 30 levels for Tharja to "catch up" to Henry's Strength lead, not to mention his weapon rank lead as well. Furthermore, once Henry starts doubling, he usually doesn't stop doubling (except on the things that most characters have trouble doubling, like swordmasters, heroes, and maybe falcon knights). To top it off, Henry has enough of a stat buffer to be able to reliably go Dark Knight, since his higher Defense makes him less reliant on Nosferatu. It's also worth noting that Tharja only has 3 more base Speed than Henry (4 if you scale on base level) and wins the growth by only 5%, which really isn't enough to call it a huge win, especially when Henry has a good claim to an exceptional Speed support.
For the most part, it comes down to Tharja's 4 maps (and what she can get out of them) versus Henry's "pretty much the rest of the game". Considering Tharja isn't exactly a blowout on those four maps, I'd say it's a wash.