Hello Pierre! I ran into the same problem now for the 3rd time (after starting the level all over again), where Oups says young Morglot's problems are now solved but if trying to wake up the adult Morglot, then Oups says "Before waking him up I think I forgot something...".
I have done all required actions with other children but apparently I don't know what is the "correct" order. When young Morglot asked Frippone to sit with him (before using the slingshot on a snake), she declined and now Morglot never asks again.
I gave teddy bear to Frippone before Morglot had used slingshot.