I remembered about halfway through building this table that a 1x6 is actually .75x5.5 so hopefully I've got all my planning numbers adjusted correctly :)
Step 1: The Plan
I wanted the trough to be about 4x4 so I planned to assemble it as shown in the picture. I put a 1x6 laying flat with a 1x6 along the back, attaching it to the table and a 1x4 making the lip in the front.
Step 2: Materials
Two 32x36x1 planed pine shelves
Four 1x4x4' knotty pine boards
Two 1x6x4' knotty pine boards
Three 1x6x6' knotty pine boards
A 2x4x10' stud
A 4x4x8' cedar post
2 steel plates (probably overkill)
Four 15x15 grey lego bases
Gorilla glue (glue down base plates)
Step 3: Table Top
Step 4: Outside Edge and Bottom
I left a 1/2" lip on the table to help keep the lego on. Then I nailed on c and d.
I flipped the table over onto the face and added the trough bottom e and f first followed by g and h.
Step 5: The Trough Lip and Test Fit
At this point the trough feels kind of flimsy, but once the lip is on, things will tighten up.
Flip the table right side up and add the trough lip L and k followed by i and j.
I had to test fit the lego bases in, so everybody is standing around looking proud after that.
Step 6: Legs and Dividers
I then measured and cut my four 24.75" 2x4s and tapped them down flush with the trough bottom before putting a couple screws in from the outside to hold them in place. You want a snug fit with the 2x4s to keep the legs sturdy.
Next, screw the 2x4s to each post.
I spent a bit of time hand shaving the corner of each post for about 6" so I could add a 45 degree support at the corners but I didn't think I needed it in the end.
I added a divider in the middle of each trough using leftover 1x4 pieces. You can see these in the finished picture.
Step 7: Glue the Baseplates On
Make sure to attach the plates together with bricks when gluing them down to get perfect spacing.
Here's a nice shot after everybody had fun building on the almost complete table :)
In hindsight, if I had glued the joints before I nailed each piece on, it would be even more kid-proof.
Let me know how yours turns out!