Upvote:2
Reapplying to the same country can help but it's not likely to bring much if you don't address the underlying concerns. And given the number of motives raised to justify the refusal, it seems this application had very serious deficiencies so I doubt your parents will succeed.
There is nothing that would stop your parents from applying to another country either but remember that all Schengen consulates have access to the same database and know that they just applied to Belgium. If the application is less than stellar, trying again immediately with a completely different story will make them look desperate to get a visa and not genuine about their intent.
The best way to get a visa in your parents' case is to wait a bit and improve their financial situations in the meantime. It's easier said than done of course but anything else is unlikely to succeed and could even backfire by damaging their credibility.