It is important that the price should be in line with the market, even if it is your children, because the state can fall back on the children if the sale price is too favorable if you are no longer able to finance your living expenses in old age and your own funds have been used up. From a tax point of view, it is advantageous anyway if you grant your children a mortgage on very attractive terms, for example. And things are much simpler with one child, because if there are several children, the distribution of the inheritance (pre-inheritance) must be planned precisely, i.e. the advantageous sale of the house to one child could be regarded as over-inheritance. In any case, consult a notary for advice.