Is it permissible to say, "May our Lord not let your efforts go to waste," when supplicating for children's success in exams, and does this statement achieve its intended purpose?

There is no harm in the phrase "May their efforts not be wasted," because it appears that the intention is to ask Allah that the children's study efforts do not go in vain due to failure. And the supplication of parents is answered, as stated in the Hadith: "Three supplications are answered without a doubt: the supplication of a parent, the supplication of a traveler, and the supplication of one who has been wronged."