There is no indication that the children are other than fully clothed or that they (or anyone else that is part of the fact pattern) are depicting or party to any lewd acts.
I think the question hinges on "they". Does the they refer to the children or those who are recording the children?
There is a decision from the 2nd Circuit earlier this year that found that 18 U.S.C. § 2251(a), which is the federal statute that mandates a 15 year sentence for “[a]ny person who employs, uses, persuades, induces, entices, or coerces any minor to engage in . . . any sexually explicit conduct for the purpose of producing any visual depiction of such conduct . . .”
The whole opinion deals with several issues, mostly with sentencing enhancements and reasonableness of the length of the sentence imposed, but the first issue would fit into your question if the "they" you refer to are the children and not the adults doing the filming. The short answer is "it could be, at least in the 2nd Circuit.
The conduct in question does not involve any lewd acts where the child is an active participant, or even aware, of the conduct itself. It involved a child, asleep, while a video was taken and shared on Kik where the defendant was visibly masturbating, along with commentary that, while superficially would have involved the child, was found to be untrue by the trial court. The video in question was described in trial court documents as:
[T]he video runs for about eight to eleven seconds, and shows Mr. Osuba masturbating some feet away from V-1, the alleged 17 year old victim. Throughout the duration of the video recording, V-1 was fast asleep on the couch with her face turned towards the backside of the couch. Throughout the duration of the video recording, V-1's face, likeness, or other distinguishing characteristic, such as a unique birthmark or other recognizable feature was never visible. V-1 was completely oblivious of what Mr. Osuba was doing nor did she wake up at any point during the entire eight to eleven seconds duration of the video. V-1's face was never seen in the video, nor any part of her skin. V-1 was fully clothed. No portion of her unclothed body was visible anywhere in the video. V-1 did not participate in any way at all, with whatever Mr. Osuba was doing. Mr. Osuba did not have any contact whatsoever with V-1. V-1 was fast asleep and was totally unaware of whatever was occurring in the room.
The district court found, and later the 2nd circuit affirmed, the conviction of this particular count, at least in accordance to precedent set in the circuit, did not require active participation or even knowledge on the part of the child so as long as "use", as commonly understood, would lead the jury to the conclusion that the defendant committed the act, and while the use of "engage" may carry some notion of active participation, the term in the specific section of the law would necessarily include "passive engagement", or otherwise the law could be construed as protecting only minors who are awake and not, using the example proposed by the court, "drugged or intoxicated" so that they cannot actively participate. Of course, those are separate illegal acts within themselves, but if a statute was written specifically with the intent to cover production of pornographic material involving children, it would make little sense that it would leave the exception open if the perpetrator simply renders the child unconscious and avoid the charge entirely.
Not all circuits necessary take up this reasoning, but so far, all circuits where this issue came up on appeals have reached the same conclusion albeit in slightly different ways. In part that is necessary because no two cases hinge on the same set of facts, but also, the law was written broadly and presumably intentionally so that it covers as much as possible. There may be a separate issue involving vagueness and as "ordinary definitions" of words are not static, it's not a given that in the future courts would keep interpreting the statute in the same manner and reach the same conclusion. But for now, it can be. Your use of "party" fits with how several different circuits have interpreted in terms of "use" and "engage" as in "being included in" or something akin to that wording, I think. As for lewd acts, the child's actions may be construed as not lewd, but the law does not require that to begin with, and that would unlikely to be an issue of contention in the future, by the looks of things. So, at least right now, it's possible, in the limited circumstances covered by the decision here and several other circuits, it's possible that a recording of a child who is fully clothed to violate the statute even if they are not participating or actively engaging in any act at all, or even aware of any act happening.