Which seems to be what they're taking advantage of here. Palestinians currently fall under Jordan's nationality laws, which dictate that
Individuals born to a Jordanian father are automatically Jordanian nationals at birth regardless of birthplace. The status is not transferrable by descent to children of Jordanian mothers unless the fathers are stateless or their nationalities are unknown. For nationality purposes, Palestinian fathers are never recognized as stateless whether they hold citizenship of any state or not.
From my limited understanding, purely because they are the children of Palestinian refugees, Belgium can't make them stateless.