All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
The question is somewhat ambiguous because as part of the details about the heirs you mentioned that the deceased has a son and a daughter, and that they both are children according to what you mentioned later, and then you stated that he gave the land to his daughter's husband and this means that he has another old daughter who was married, other than the two children. So, if he has another daughter and she was alive when he died, then she is among the heirs.
However, we will base our answer on the assumption that the deceased left only one son and one daughter, and that the other daughter had died before him.
Hence, we say that if the heirs are only his son and his daughter, and the deceased father did not leave any other heir, then the inheritance will be divided between them by Ta’seeb (by virtue of having a paternal relation with the deceased and not having an allotted share, so they get what is left after the allotted shares have been distributed), with the male getting twice the share of the female, as Allaah says (what means): {Allaah instructs you concerning your children [i.e. their portions of inheritance]: for the male, what is equal to the share of two females.}[Quran 4:11]
Therefore, the inheritance should be divided into 3 shares, the son gets 2 shares and the daughter gets 1 share.
As regards the land that the deceased had given to his son-in-law before his death, then this has to be investigated whether he had given it to him as a gift, or as a trust. If it was a gift, it should be investigated whether he had gifted it to him when he was in good health, or during his death sickness, and whether or not his son-in-law had actually possessed the land? Also, do the heirs and the husband of the daughter agree that it was a gift or a loan, or do they differ about this?
All these probabilities have an effect on the answer and it takes a long answer to mention them all. Therefore, we recommend the heirs to ask the scholars orally about their case so that all disputing parties will be heard, and in that case it will become clear who is the one who has the right to have possession of the land.
Allaah Knows best.