a soft giggle
A soft giggle is when you laugh like a little kid, but it's too quiet for anyone to notice. It’s like a whisper of a laugh.
He did a soft giggle when I told him I ate his sandwich. He was trying to hide it.
She did a soft giggle when I said I was a wizard. She didn’t believe me.
I did a soft giggle when I saw my dog wearing a hat. It was too cute.