can't the children buy the parents' house without real estate transfer tax?! there must definitely be tricks like gifts or something?!
Yes, a sale to one's own children is exempt from real estate transfer tax! No tricks are needed here. But this is a sale in Bavaria anyway, so 3.5% (but it's supposed to increase to 6% real estate transfer tax).
As of October 2015, it may still be delayed because it has not yet passed through the [Landtag]. In Brandenburg, the date is already set for the increase from 3.5% to 6%. Berlin now wants to raise it to 6.5%.