Translation:On Saturday, we were sitting in the theater behind Žofie's mother.
Well, actually, there may be a subtle difference between
"V sobotu jsme seděli v divadle za Žofiinou matkou. "
"V sobotu jsme seděli za Žofiinou matkou v divadle. "
In the first one the new information that you were siting behind Žofie's mother and the fact you were in the theatre is probably already known.
In the other one you first convey you were somewhere seated behind Žofie's mother and then you localize the place to be the theatre. The fact that you were seated behind her somewhere may be already known.
I was not implying it shouldn't. My first comment was "I don't think so." and only after I added there might be a difference in Czech. However, the ordering of adverbials in English is different and must be assessed by native English speakers.
Do note that Denise did not ask whether one or the other should or should not be accepted.