I decided to go with 4/4 red oak mostly because I like the grain pattern.
The front and back will be roughly 3×34.5 and have a mortise in the middle with the ends cut to accept the other portion of the center finger joint.
The sides will be roughly 3×35.5.
The middle support will be 2.5×36 and contain the tendons. I wanted to make sure the middle section sits flush with the 2 plywood tops and the sides have a small lip for kids to hold onto.
The legs are made from 2 pieces to hopefully give them a bit more weight and stability.
The two plywood tops will be made from ¾ birch (my wife wants this painted to make it a little easier to clean/ repaint if needed). As a side note, my 2 year old daughter already learned that Sharpe markers are indeed permanent :) The dimensions will be about 34×16 1/8 for both of them. I also edge banded the sides and put the lego plate on one face.
Finally, several cleats will hold the tops in place as well as hold the “lego bins” in place. I made these from ½ MDF.
Four ¾ x 26.5
Four ¾ x 9
Six 1×16 1/8