hmm I think your zoo needs some work, penguins eat krill or fish, not seal...
seals tend to eat cephalapods and crustaceans (Ie not just fish, though they do eat those)...
walruses have been known to eat seal, but that is rare..
link for more mainly they eat 'bivalves' like clams.
but hey now your DD has lots of questions (like can I change the pecking order for people too?)