Your sentence is fine, it is what someone from the UK would say. One sweet, two sweets, three sweets etc. 'Candy/candies' to us is an Americanism. Of course we understand it but you have it right, we say 'sweet(s).' Or little children might say 'sweetie(s).' 'Treat' sounds like something you give to a dog. Best avoided with human beings.

From a British perspective, your example sounds fine to me. One sweet Two sweets A packet of sweets

"Sweets" wouldn't be used in this context. In the case where you give somebody something, you would say "I gave the boy a cookie and the girl three cookies." But you might say I ate some sweets before dinner. It is a general term. Hope that helps.

I gave the boy one sweet and the girl three sweets is fine In the U.K I gave them her/him/a-the/boy/girl etc A sweet 3 sweets packet of sweets tin of sweets box of sweets

There is no difference between American use and British use regarding singular and plural you just swap words "candies, treats, cookies" for America and 'sweet, or cookie(biscuit)" for Britain​