when supporting adult children, can it be taken from maaser funds?
if not, what if they are a kollel couple?
if yes, does it only qualify if they are using it for bare necessities; what if they use it to buy a house?
Assuming that the couple indeed don’t have enough for their own expenses etc. it is permitted and a mitzva to support one’s children. The Midrash says that we learn from the verse “ umbsarcha al tisalem” -and from your flesh don’t be indifferent” that we have a specific obligation to support our relatives. The Shulchan Aruch gives a list of preference of whom to give one’s maaser first, and children are almost at the top of the list (after supporting oneself, parents and Rebbi). This is even if they are not in kollel, but need the money. How much ore so if the child is learning torah, then not only is it tzedakah, it is also supporting torah, which is also from the greatest forms of tzedakah. Included in this is whatever their needs are, and if they have a need to buy a house, that is also included.
Tanna Dbei Eliyahu 27, Rambam Hilchos Matnas Aniyim 7-13, Y:D 251-3, Gra Y:D 251-4.