Calling jdbcMutableAclService.deleteAcl(parentOid, true) should remove the parent acl and, also, its children. The records are deleted from database, but the children related entries from cache are not removed.
The element in cache was built using an ObjectIdentityImpl that received as parameters a string as class name and a Long as domain object id. But, calling JdbcAclService.findChildren() the domain objects returned are created using two strings as parameters and because of this the children are not found in cache and, thus, not removed.
A possible solution could be the creation of ObjectIdentityImpl objects in JdbcAclService.findChildren() method using a Long as the id of the domain object.
The test method demonstrating this behavior is found JdbcAclServiceTests - testDeleteAclAlsoDeletesChildren.