You can access the island in the Bois de la Cambre in different ways.
All of them require you to cross the lake, so you have to go to the pier to get to the other side. The ferry costs 1 euro per person (round trip).
There are several parking areas
near the Bois de la Cambre.
Get off at the Brésil stop.
Then walk through the park for 10 minutes, to Chalet Robinson.
you will have no problems with traffic jams or parking!