Hi,
are you going to travel by train from Amsterdam to those parks ? I don't really know how good/bad the connections are to the Dutch and German parks. For Belgium, it will be a +3 hours journey till the gates of both parks. For WB, take a direct train fm Amsterdam to Brussels and from there a rail connection to the station near WB (Ottignies I think). For BJL take a direct train to Antwerp. From there the train to Herentals and then the bus to Bjl. Ask the driver where to get of as you will not actually see Bjl at the busstop. It is still 15 minutes walking from there.
If you're renting a car, make sure you hire a gps And then I would suggest Toverland aswell. Maybe you can combine Toverland and MPG or Toverland and Bjl on one day in that case, although MPG and Bjl are more than half-a-day-parks. Be warned about schooltrip crowds on weekdays in the second half of June !
Duinrel and Drievliet are also 2 parks not that far from Amsterdam and they can be combined in 1 day (by car that is)