I translated Tiere as pets and it was marked wrong! I'm confused.
Pets are "Haustiere" (house animals), "Tiere" means animals.
Is it wrong to translate "kind" to "boy"
"Kind" means "child", so it can be a boy or a girl. That's why yes, it is wrong.
Why does not Tiere go with "die" here?
Because there is no the in the English sentence and therefore no definite group of animals being referred to that would require the definite article to be used.