Casual loanword for a child or young person, similar to English 'kid'. Often used in informal contexts.
あのキッドはサッカーが上手だね。
That kid is good at soccer.
近所のキッドたちが公園で遊んでいる。
The neighborhood kids are playing in the park.
キッズ is the plural loanword used in marketing (e.g. キッズ服 'kids' clothing'), while キッド refers to a single child.
counter for people
Borrowed from English 'kid'.