Need a foundation for my picnic tables, at present on grass,

Here are several ideas:

-put in a cement pad under the picnic table

-pavers or gravel

-a huge sheet of plexiglass

Put down some of the patio blocks which come in various sizes. You should remove the grass first, lay down some sand, tamp it down and then install the blocks.

Have you thought of pavers?

If it were me, I would use flat patio stones you use to make a patio on the grass. Just put one under each leg for support and get them level

If you want to keep your lawn, take up the grass where the 4 legs set, dig down the depth of a concrete paving square you can purchase at the big box store, plus about 1/2". Fill square dug out area with sand and level paver. The area around these pavers and under the tables can be cut with a weed eater.

If you want a large area that is without grass, you will have to put down a paver patio.