Training exercises, technical knowledge, and theory learned in books are just tools a player uses during a game. However a lot of what goes on during a game can only be practiced during a game, which I'll try to sum up as "over the board analysis"... clean calculation and reasonable evaluations on the clock.
For a newer player (like those kids) it may be more about discipline to check how the opponent's pieces may refute the intended move. e.g. they're so focused on preparing the great positional pawn break they learned from the book that they neglect the common pitfalls of basic tactics that will constantly try to undermine them.
Long and short of it, it takes time to integrate these new tools into the game playing process. A process that involves a lot of practical skills like judgement, visualization, and of course blunder checking.
I started playing on this site in May, after a dozen years off from chess (other than sporadic games at another site.) I decided I liked it here, got a premium membership, and began using the features to "work on my game" - tactics, brushing up on endings, looking at some sound and sensible openings (for a change!)
Needless to say, after little over a week of that my playing went kaput - a 100-point rating bath in about a week. No conception at all about what was going on in games. Overlooking even the most obvious two-movers, etc.
This isn't the first time this has happened in my chess life - work at improvement accompanied by some Gawd-awful playing. I'm seen it in other players, too; in the '90s I did chess programs at 3 schools. Some kids would decide to "get serious," start working methodically at their chess - and go through a spell of terrible results.
Fortunately these were shortlived, and most of those kids would follow up with a big surge in playing strength.
I have some ideas about why these bad streaks happen, just when we "should" be playing better. (But those ideas fail to explain how you can be working-productively- on tactical exercises and suddenly be unable to see the simplest tactic in a game!) How about you? Has this happened in your chess? Do you know why?