Hi Cathrin! Technically you could as a nickname... but it's not used often. For example, when you use sweetheart as a nick name for your boyfriend or girlfriend you say, "Hi sweetheart!" Saying, "Hi sweet boy/girl!" though, is rarely used.

That phrase IS used often to describe someone, though. For example, you can say, "She is such a sweetheart!" OR "She is such a sweet girl!" and they would mean the same thing.

Does that help?

To be honest, and I know I'm speaking on to of my head, but a lot of times it's not even gender dependent. The only one I can think of immediately is 'babe' when used towards girls. You could just call them 'sweetie'. Some colloquial ones would be 'boo' or 'boo thang'.

If you mean you want to call someone like this romantically,  then no.  You can call a young child sweet girl / boy though. 

Hello,
Myself AARKAY from India was delightful seeing your profile & feel to be added as friend

into your list. I would wish maintain contact for better English communication skills

practice.
Waiting for your kind attention please.

Regards.
skype: rk.spkezy

I got it. Thank you. But if I need to emphasize that I refer to girl or boy, what expression can I use?