According to this tweet, the Belgium government has now issued authorization for theme parks to reopen starting June 8th, 2020, as long as they use a list of proposed new operational guidelines. The list includes social distancing, reduced capacity attendance limits, closing indoor spaces and new cleaning standards. The June 8th date is contingent on if conditions are met within the country during prior phases of business reopenings set to take place over the month of May. These prior phases will see a return to the workplace and the reopening of certain classes of businesses at different times (May 4 and May 11), a gradual return to school operations, family visits and allowing new travelers into the country (May 18).
Some of the most popular parks in Belgium include: Bobbejaanland, most of the Plopsa parks, Walibi Belgium and Bellewaerde.