EPCM source filter - ways to exclude member

We are using Enterprise PCM.
We have a Entity hierarchy where Parent1 has 4 children. Child1 & Child2 are not level-0 and Child3 & Child4 are level-0 members.
Paren1
——-Child1
—————Child11
—————Child12
——-Child2
—————Child21
—————Child22
——-Child3
——-Child4

In the allocation rule, we want to calculate Parent1 less Child2.
What options we have to achieve this in the source dimension filter.