The river crossing riddle is described in this video (it’s okay to play it, it will probably stop before revealing the solution):

The rules (from the last part of that video):

  1. The raft needs at least one animal to paddle it across the river, and it can hold at most two animals.
  2. If the lions ever outnumber the wildebeest on either side of the river (including the animals in the boat if it’s on that side), they’ll eat the wildebeest.
  3. The animals can’t just swim across, and there are no tricks; the animals have to use the boat as described.

I wanted to figure it out, so I made a simulation. And I chose this particular lion picture because I’m pretty sure it’s the lion picture my daughters would choose: