You should take them back to the vet for another checkup.

The vet is right in that they need meat, and that dry food is bad for them.

Try feeding only raw or cooked chicken (boiled, no bones, no spices) for three days and see if that helps. If it does, then it's clear that they're sensitive to dry food and need to have wet food only. Good brands are HiLife pouches, Bozita, Applaws kitten - all these are wet foods with more than 50% meat.

Hope that helps.

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