A property given to a child while the parents are alive is a "gift." A property given to a child after the parent's death is an "inheritance."
"Gift" and "inheritance" given by parents to a child are separate property of the child. So they do not have to split them with their spouse when they divorce. Usually, there are many misunderstandings regarding this.
First, when the parents give property to their child before the child's marriage, they do not need to be split with the spouse in divorce. However, if these properties are mixed with marital properties, they can be subject to property division in divorce. When the child spends time or his income managing the separate properties after marriage, some portion of separate properties become marital properties like a drop of black ink spreads over the water: this is what I mean by separate properties being mixed with matrimonial properties. Let's say the parents gave their son an apartment before his marriage, and he managed it by himself after his marriage. In this situation, his wife supports his labor of managing the apartment. So his wife can claim her right to increase the asset value of the apartment. When this happens, many clients question, "Why does his wife have any property right to the apartment?". This question comes from a misunderstanding that their sons' labor is his separate property. So, to my readers, please remember that the child's labor after his marriage is marital property. It allows his wife to claim a property right to have a portion of his separate properties—if he manages them after their marriage. Indeed, to avoid this situation, you can make a prenuptial agreement that states that the wife cannot claim any right to the husband's separate properties, which you give her before their marriage, and let her sign it. But when you make a prenuptial agreement, I strongly encourage you to find a lawyer specializing in family law and consult the lawyer to make a prenuptial agreement. A long time ago, my client brought something like a pledge form that, he claimed, was hand-written by his son's wife before the son's marriage. However, it does not meet any terms and conditions for legal effect. Unfortunately, I could not help but say that the form cannot provide any legal help, although it gave you some emotional comfort.
Second, even when properties are given to a child after marriage, the ownership of the given real estate, company, or current assets belongs only to the child's name, so these separate properties are not subject to property division in divorce. There are many misunderstandings about this as well. The properties given by the parents after the child's marriage are primarily the child's separate property. If the parents gave their daughter real estate, this property is mixed with marital properties if she spent any of her or her husband's income paying the mortgage, property taxes, or repair fees for the property after marriage. Some clients may ask, "Only my daughter works, and her husband takes care of children. But why can her husband claim any property right to her separate property just because my daughter paid the mortgage after marriage?" But I already explained that the daughter's labor after marriage is a marital property at length.
Thus, to keep the property you gave to your child separate, the child should only passively gain benefits from the property. You must remember that no labor and effort should be given to the property, and you should make an account only for the given property to maintain it as a separate property. Also, you should consider whether giving your child the estate is the best idea with an expert.