Two only children is a literal rendering; the extra wording in Good News Translation (“the only children of their parents”) is to help readers who may not know the English phrase “only child.”

Be merciful … keep them safe: See 6.17.

O Master: See the note on 3.14.

Bring their lives to fulfilment need mean no more than “let them live out their lives.” No purpose is necessarily intended.

Happiness and mercy is closer to the Greek than “happiness and love” (Good News Translation). The two qualities of happiness and mercy may be linked: “happy in your mercy/enjoying your mercy”; these two lines may be also expressed as

• As they live out their lives, may they be happy because you are merciful to them.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Tobit. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here.