Outline the Characteristics
Its important to remember that when you begin a project you should outline the characteristics of the end result. Is this for a child? Are you planning to paint the surface? How many do I need? All of these questions and others are very important in defining your lumber needs for your project.
Hard Wood Vs. Soft Wood
As a general rule, hard wood is heavier and stronger than soft wood, but is also more expensive. However is hard wood always the best choice? For toys and blocks for children, generally you will want to go with a harder wood that is not going to end up breaking under the pressures of child play. However if you are looking to do arts and crafts, you don't need to have the strongest wood available or the more beautiful grain color because these blocks will not receive the same rigorous treatment as children's toys.
Sanded Edges Vs. Not
Children's toys are meant to be well constructed, educational and safe, so the best choice for anything a child will play with is to have the edges Sanded. Straight edged corners are more artsy and provide a desired formal look.