i wouldn't worry too much about 'working tax creds' becuase you have a legal right to claim them, you need to show bank statements with money to support thier visit.
more likely they will be refused on 'the balance of probabilties' they will not return to the phils.
so show they have reasons to return to the phils.Example thier brothers, sisters, sons, daughters, grand kids( a simple family tree- showing all thier familiy are in the phils,thier home, land, pension, savings,friends etc.
