Is 'She trod on' wrong?
No. If Duo says otherwise, report it.
do natives say "the toys of the children"?
des enfants is plural so it's kids' toys not kid's toys. Tut tut.
"Kids' toys" = "children's toys". Silly Billy!
"walked on"was marked wrong
Since "des enfants" is used, shouldn't the translation be : "She stepped on the CHILDRENS' toys"?
No. The plural of "child's toys" is "children's toys". "Childrens' toys" does not exist (except as an error).
I agree with Michael. 'Stood' should be accepted. In UK we would not say 'stepped on'.
Both are in common usage in the UK.
"She walked on..." accepted