I've been bringing Silas to my studio since he was a toddler and letting him sculpt and build with scraps of steel and wood. Now that he's in primary school, he has started bringing his friends over to the studio for playdates.
I give the boys a box of scrap metal pieces and leftover parts and supplies and let them pull out pieces that they want to use for their own sculpture. I always place a piece on the table for each boy and ask him to work on building his sculpture around it. This gives the boys a focal point so that there's no frustration in how to begin.
After the boys have decided on their design and show me how they want the pieces assembled, I weld everything together for them.
It's never too early to introduce kids to design in different mediums (no offense to Legos).