How does it work?

Do you have to feed it to each pet 10 times to fill them per day, or..?

Feeding it once gets your pet to 10/10 hunger. It decays the same as if you had fed it 10 food items. You have to feed it to each pet you want full.

'Decays' as in, it's gone after one use?

No, the item is not consumed but your pets gets hungry again at the normal rate. Just feed it the same berry next time you want it full.

Thanks :)