As far as the father is alive he alone is the owner of all his properties. He can give it to whomever he wants to give, but with the condition that he does not cause any harm to his children. It is not right to violate the right of one’s legal heirs. It is mentioned in a hadith that whoever cuts the right of one’s heir Almighty Allah shall cut his right from the Jannah. Hence if he has given his children as per their requirements then he also can give his grandsons as gift.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband