My two cents on strength and wood selection. Unless the design is weird or needs specialty engineering, most domestic softwoods and hardwoods will handle typical furniture loads, so the design is more important than the wood itself. You can think of this as the structural strength (that is will it collapse?). The weakest link in most designs is usually the joinery. Make sure you do them correctly.
You might want to consider a hardwood. There is another aspect of strength that has nothing to do with the structural strength. How well will the wood stand up to the abuses of the users - dents and dings and so on? Poplar and pine will get dinged up more easily than oak or cherry.
Of course, softer woods will be a little easier on the inevitable banged head. Also, make sure you break all the edges and avoid sharp corners as much as possible. Iíve put a few miles of stitches into kids after a kid vs furniture encounter. Itís a pet peeve of mine, so I donít mean to preach, but Iím often amazed how often I see designs for kidís furniture that doesnít consider the likelihood that at one time or another the children will bang into it. The furniture usually wins.