Anybody know what the plant with the larger round leaves might be? My son bought a bag of "small pond plants" and I was able to ID the duckweed, and water fern for him, but not sure on the larger one.

Looks like frogbit to me

Yep, looks like frogbit.

In my ponds these three cannot live together. The azolla (water fern) doubles in size every week and completely swamps the duckweed. Relative competitiveness probably varies, though, based on which species of these three you have, and water, air, and light conditions.

Fish will change the equation, too, of course.