I think you may have put someone else's homework in my daughter's backpack by mistake.
It looks like someone may have given my daughter another kid's homework by mistake.
ーI think you may have put someone else's homework in my daughter's backpack by mistake.
「娘のバックパックに別の子の宿題が間違って入っていました」
by mistake で「間違えて・誤って」
先生が誤って入れたと思うなら、I think you may have put ... by mistake. のように言うと良いでしょう。
ーIt looks like someone may have given my daughter another kid's homework by mistake.
「誰かが誤って娘に他の子の宿題を渡したようです」
誰かわからないけど、誰かが誤っって入れたと言うなら、It looks like someone may have given ... by mistake. のように言うと良いでしょう。
ご参考まで！
カナダ 
