Is it permissible to give one's parents from the obligatory charity (sadaqah wajibah), knowing that it is not possible to combine giving charity with providing assistance to them?

Giving charity to needy parents is better and brings greater reward from Allah, as it involves showing dutifulness to one's parents, and they have a greater right to your assistance than others. Start by fulfilling their needs; if anything remains after sufficiently providing for them, then you may give it as charity if you wish.