Stop measuring.
Stop fiddling
Put them in a shaded spot out of the wind. Avoid the soild being completely wet. And leave them alone.
You gave them open heart surgery at a very bad time of the year. Best would have been to just leave them till winter, as recommended. Now you just have to wait and see. Every time you fiddle with them, they have to balance a new reality. Plants do not respond n timeframes of hours, but days to weeks. SO let them react.
ONLY thing you can consider is using a humidity dome of some sort. But that seems to be unneeded. One seedling is deciding to die. The rest will probably make it as long as the roots do not drown and you do not put them back into the sun untill well established..
I measure because I want to document the whole process, successes and failures, both for my own sake & sanity, and for anyone else who decides to dive down the rabbit-hole after me.
I haven't trimmed or the likes since taking the extremely dead leaves off the tallest seedling on Friday, the extent of my interaction has been monitoring the soil (it finally needed a proper watering today as it was getting pretty dry), misting the leaves occasionally when the day's at its hottest, and having moved them to the porch w/ some subsequent position adjustments so they aren't roasted by late-afternoon sunlight.
Been doing my best on the position & soil maintenance front - they live out on a screen porch right now, and I've been keeping them out of the path of the late afternoon direct sunlight so they don't get roasted. Soil has seen very little attention since the first post-transplant watering, as I know it's wiser to err on the side of letting it come close to drying out so as to avoid root rot.
As I think I'd said in the original post, I wasn't sure they'd make it *to* winter - they were all growing out of river rock w/ no real access to actual soil unless they broke through the ground cloth underneath (which the tall guy did, hence its unsurprisingly more traumatic transplant experience) - and I'd been under the impression that trees this young were generally more amenable to transplants year-'round, in particular from people like HorseloverFat & Sorce sharing their own experiences in transplanting non-ideal trees at non-ideal times of year. That impression may very well be wrong.
I've been doing my best to let 'em be and watch how they react, keeping them out of the path of sunlight notwithstanding. If this, too, is a dumb idea & I should just plonk them down and let them be, even when they get a few hours of sunlight through the screen, let me know - I'm erring on the side of more paranoia rather than less about leaf scorch, since I know it's a real concern.
(Edit: I should also mention - I'm not trying to throw anyone under the bus! I don't blame anyone here for what's happening to the seedlings but myself, nor do I think any of the advice offered has been bad - y'all rock & are sharing your experiences & techniques, and I'm inevitably going to fumble it for a while until I've had the opportunity to learn & practice more as a patron of the tiny forest.)
Humidity is semi-decent here, and I've got a spray-bottle on hand & mist the leaves when it's really hot to keep them from drying out. I assume there's very little I can do for the tall guy but keep it lightly watered, out of shade, etc, and care for it like the others in the hopes it changes its mind and bounces back?
As
@leatherback said, you are loving them to death. The best way to stop that is go get more trees. When you have so many trees you can’t quite work on all of them every year, you have about the right amount.
I realise there's very little I need to do with it right now, but this is part of why I decided to grab that little Ficus at Lowes yesterday, since it was, A, not very expensive (like $30ish IIRC), B, in a style & size-range I liked, C, a significantly more newbie-friendly tree species that can be kept indoors, and D, it's another thing to work on and keep my mind and attention elsewhere from the seedlings.
We're gonna be heading back next week likely to see if they've gotten their perlite and the likes back again, and hope to peruse the regular young trees, see if anything calls out as interesting material to work on, so as to add yet another source of distraction for tiny tree maintenance.
IMO this is a perfectly fine frame of mind as long as it's not the only thing you're doing.
I.e. you should also be leveling up your game by also cultivating the "suitable" species for experience (of which Acer ruburm is not one). That way when you encounter a problem with your sentimental seedlings, you'll know what to do, know what's normal and what isn't normal, etc, etc.
To put it another way, you're just setting yourself up for failure if you try to tackle a very hard species with limited experience.
I do plan to try other species, just need time, money, and opportunities to do so - as outlined above there was a Shohnin-sized formal broom ficus of some description (ginseng, maybe?) that I grabbed on a Lowes trip yesterday, and I'm hoping to get some regular young trees there to fit into the prebonsai/in need of training tree progression slot, as well as to learn more about working with other kinds of more popular trees for bonsai purposes.
==END REPLIES, BEGIN MORNING STATUS UPDATE, DAY 4==
Things are more or less the same as they were at the start of yesterday, maybe slightly droopier as today's started off warmer & they've been a bit farther from the brightest spot on the porch so as to avoid them getting roasted by afternoon sunlight as occurred for an hour or two yesterday before I realised what was happening. Also got the label off the ficus, and checked the soil water level - still a bit damp, but it might need watered this evening as it's definitely drying out. Need to track down a plate or something to use as a temporary tray for the pot to drain onto. Pictures incoming, any commentary will be in the alt-text as usual.
(Postal Note: the Lowes' plant page for the ficus literally just says it is "a bonsai", and while it does provide care instructions that more or less fall in line with my understanding of how to care for ficus bonsai, it doesn't actually specify species or the likes. Literally says "Scientific Name: Bonsai" - ace documentation, Lowes. </sarcasm>)
