I'm looking to make a 40" by 28" table for a six year old. I would like to use 1/2" plywood for the top and for two shelving units attached below. Those shelving units will also serve as the front two legs, one on each side. Would 1/2" be sufficient for strength and stability of the table? I would rather not use 3/4" only because I would like to be able to move it more easily.
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 & 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! :)
Thin plywood is likely to be too flexible by itself. I think you'll want to run a couple of boards across the space under it, at least. Remember that any kids' furniture is eventually going to be climbed upon; you don't want to underdesign it.