Mau should be accepted.
Mereka hanya ingin jalan-jalan should be acceptable
"They just want to take a walk." Translation:Mereka hanya ingin berjalan-jalan.
Mereka hanya mau berjalan-jalan.=not accepted, reported
Salahkah "mereka mau berjalan jalan ajah"?
"ajah" is too informal. However, I guess "Mereka mau berjalan-jalan saja" should be accepted since it has the same meaning.