It would not be THE end of Myriam/Marc dom path, there will be other way to go, it will be one possible end.
Well, I'm not big on the shota/loli (though I'm kind of coming close to it with the kids at the river).
Well, egoistically
, I am following more than one path in parallel, and in one of the paths (or two) I may actually branch exploring different ways to develop on the Myriam/Marc relation (and even to kind of correct it, so really, all the possibilities), so, it is interesting to know there will be way to go forward on that.
About the kids at the river, well, playing "devil's advocate" with psycobabble
... Marc has already tried (and succeeded, depending on the choices) to exploit the mothernal aspect, to get something, and Myriam has shown being potentially sensitive in that area, especially if there was also some build up of the submissiveness and the two combine..
So, could play also the element of "helping the young ones safely explore sexuality". If she has "trust" high enough (if I understood well how "trust" works), that could also include some made up story by the kids to make her feel more compelled to help them, though it would not be strictly necessary (maybe could make her more keen, again a bit like Marc tries to make her feel responsible/guilt). There is also the possibility of following up on the fact one of the choices was to ignore them, bringing to a +1 in submissiveness and making them potentially willing to try again, and being a bit more forward, next time. Aside the "flashing", that even if concluded with scaring them, could make them hope to get the same or a bit more next time.
All those could bring to more than one interaction over time, and more than one possibility of blocking completely, blocking temporarily or developing/excalating further (a bit like happens with Marc, William, etc.), also given Myriam has already indicated she loves to go riding, plans to do it often, and she has the horse in the stables. In which direction that development/excalation goes, to be seen by you and by the player choices/variables values.
Others already mentioned blackmail as a way to make things go "lew" with the ones at the river, but in itself, I am not sure how much effect that would have (even in the most exhibitionist case, the picture they took at the river was not much, and it would be kind of visible that was taken as a kind of sneaky picture), unless further developed.
Other ways to reach that ould be either Myriam becomes really a "complete nimpho does everything alive"
like lust 600
, and then even they are a go without any restraint. Or that she feels so desperate for attention that she will enjoy feeling attractive for them ("I can still get all ages to fantasize about me") and "reward" them.
Though I admit I see the first one as really extreme, and the second one as rather forced - as she gets freer, she can see she can get attention anyway.
Other harder scenarios would seem not working for them, they are sexually curious, but they do not seem the aggressive/violent bunch, unless their behaviour changes like Marc at a certain point, or in their explorations they start to escalate by seeing something that "inspires" them, or Myriam herself suggesting something.
Unfortunately, real life chronicles say there are 13-14 years olds who do group rape, even if often in groups also with someone at least a couple of years older, e.g. 17, and most often to someone in the same age range or like early twenties - but that is not normal standard behaviour, while curiosity for the other sex is pretty normal.
Diffferent story for the bullies in the underpass who are older, especially their leader, can certainly be tempted to go directly for a more violent approach (he basically threatened her with that) as group, also to reassert himself as the leader in front of the others.
About the age... Actually, I have myself a problem with the depiction of characters that to my eyes seem too young, even when the games says are 18 or one can set the age, so I think I can understand you.
But in that case, you put them in story in a somehow coherent and kind of realistic way (starting to look for, and curious about, sexual stuff), and they are substantially males taking the initiative with an older woman, so, I don't have a problem, also considering it's only a game - but of course that is just my opinion and support for you if you go forward also with that line.