Is 1/2" plywood sufficient in strength as a 40"x28" table top?
Is 1/2" plywood sufficient in strength as a table top 40"x28", given an appropriate support structure beneath? Absolutely. 

That being said, you want to make sure that your design incorporates support in multiple axes of stress as well as twist &amp; shear. 

Avoid large overhangs with such thin plywood, and consider building up the edge with multiple layers to decrease flex.

Rabbets (rebates for the UK folks), dado's (dadoes?), a pin nailer, screws, glue, some trim.. epoxy coating for durability.. 

Heck, include a pocket or a chute along the back to sweep loose lego into and you'll have one happy 6-year-old! :)