I agree, they do need a bigger brooder. It doesn't look like they have anyway to get away from the heat if they needed to. You could make a large enough brooder one that all can fit.
I used a similar tub for 2 polish chicks and even as small as they are they outgrew that fast.
With that said, I don't think that's why your ducklings died. Other issues will definitely arise with that set up.
However, what type of shavings are you using? It looks like cedar to me. Although some use cedar, I wouldn't for young birds.
Also, the feeders and waterers aren't good for ducklings.
This is a sticky in the duck forum:
https://www.backyardchickens.com/threads/ducks-sticky-topics-index.256233/
Look in the caring and feeding of ducklings - there are examples of feeders and waterers. Many just take a plastic container with a lid and cut holes in it so that the duckling's head can fit, but not body.
@Miss Lydia has good examples.