So when they returned to their father, they said, "O our father, (12:63)

﴿فَلَمَّا رَجَعُوا إِلَىٰ أَبِيهِمْ قَالُوا يَا أَبَانَا مُنِعَ مِنَّا الْكَيْلُ فَأَرْسِلْ مَعَنَا أَخَانَا نَكْتَلْ وَإِنَّا لَهُ لَحَافِظُونَ﴾
[ يوسف: 63]

12:63 So when they returned to their father, they said, "O our father, [further] measure has been denied to us, so send with us our brother [that] we will be given measure. And indeed, we will be his guardians."

When they returned to their father, and told him about the generosity of Joseph to them, they also told their father that any further measure would be denied to them if they did not bring their brother with them.
They asked him to send him with them, because if he did so they would be given provisions.
They promised their father that they would take good care of him, bringing him back safely.

So, when they returned to their father, they said: "O our father! No more measure of grain shall we get (unless we take our brother). So send our brother with us, and we shall get our measure and truly we will guard him."

phonetic Transliteration

Falamma rajaAAoo ila abeehim qaloo ya abana muniAAa minna alkaylu faarsil maAAana akhana naktal wainna lahu lahafithoona

Abdullah Yusuf Ali - Translation

Now when they returned to their father, they said: "O our father! No more measure of grain shall we get (unless we take our brother): So send our brother with us, that we may get our measure; and we will indeed take every care of him."

Safi-ur-Rahman al-Mubarakpuri

So, when they returned to their father, they said: "O our father! No more measure of grain shall we get (unless we take our brother). So send our brother with us, and we shall get our measure and truly, we will guard him."

Page 242 English transliteration