To get across, you can climb and walk on top of the third tier (there's a narrow walkway surrounded on both side with walls), but we opted to skip that and headed across the lower level and towards the river, so the kids could have some time to splash around.
The river was busy, with lots of swimmers, kayakers, people diving into the water from short cliffs, and people just lounging about. The beachy area was a mixture of rocks, grass, and dirt, and there were more than enough places to go that it didn't feel too crowded. We picked a place pretty near the entrance, where it was shallow and there were lots of other kids playing. The river itself is cool, but not cold, is shallow for a while before getting deep, and has a rocky bottom that was full of little fish who were quick to swim away.
We saw a lot of families with kids young and old, but honestly I was not able to communicate with anyone. This was for a number of reasons, such as: a) the parents were in the river and their kids were playing by the river bank, b) the kids were older, which meant the parents were relaxing or chatting amongst themselves, c) the parents were too pre-occupied running after babies who were trying to eat mud :), and of course d) my French isn't exactly up to par for just striking up a conversation.
So we just enjoyed each other, and the simple world of children playing in the water.