Isn't this the cutest idea? A toddler sized hammock under the table via Joyful Abode.
If you like this idea but wanted to try and make one without impeding the top of your table you could make your own under-the-table hammock with rope and hardware. See the how-to below via DIY Gear Supply. I'd secure the ends of the rope to the table via two eye bolts drilled into the sides of the table. Of course, try at your own risk.