In your case argument, no one can disagree if a kid's life is at risk without good cause.  It would then have been inexcusable should the mother have continued on the flight with the toddler not seated as you would require.  While the airline is liable civilly for monetary contract damages, your logical reasoning then compels that the mother be criminally prosecuted for neglect or similar if she had put the kid's life unreasonably and inexcusably at risk.  If it is as serious as you claim, then hopefully, here, the mother deboarded the plane with her child?  If not, then perhaps a complaint should be filed against the mother?