I think it's a little more nuanced than that, at least for some.
Yes, the team is struggling out the gate, in no small part because their goals against has been atrocious. Not the ideal time to have rookies learning on the job, I'll fully admit.
Are those circumstances their fault, though? The Blues were running a less than ideal system for awhile and everyone was looking terrible defensively during that stretch. I put that on the coach.
Then the coach adjusted the system and the team started playing better, but they still found a way to lose in games they were competitive by making a few key mistakes at critical times. It wasn't the kids making those very crucial mistakes, so again, are they to blame?
Heck, Thomas has only been on the ice for two goals against this season, and Kyrou/Blais for four. As a comparison, Maroon has been out there for 11 ES goals against and Tarasenko for 13. The Blues have given up 39 goals this year, to put those numbers in perspective.
Caveat: I'm not sure what overlap there is between those goals against (i.e. if Blais/Thomas/Kyrou were out there as a line, each would have a goal against for a total of three by these counts, but it would only represent one of the 39 goals against the Blues have, so the apparent impact on the team would be overstated by the numbers I'm reporting relative to the actual impact). Since Kyrou/Thomas/Blais have all spent significant time on the 4th line, that's a real concern. Either way, they're not exactly getting lit up.
Once the team's back was against the wall, it seems like Yeo didn't really feel like he has the luxury of letting the kids learn on the job. I get that, but it's generally not the kids' fault the team is in that position.
If you think the NHL is a better developmental path for Thomas and Kyrou at this point, as I personally do, and if you think there's a chance they could develop into (relatively) useful players by the end of the season, then I feel like there's some room for displeasure about how all this is playing out. Displeasure with where the team currently is in the standings, and with the corner the coach has backed himself into with regards to some of the youngsters he might otherwise be able to cultivate at the NHL level...something the team itself seemed predisposed to doing when camp broke before everything went south.
We'll see how it all plays out, but at some point this team needs to suck it up and break in the guys who are most important to their future. Not give them a taste...really commit to their development at this level and break them in. The longer they find (or back into) reasons to put it off, the longer it will be before the kids will be making a difference. This core won't be together forever, so the clock is ticking.