Like a child in a sweet shop British expression
Very pleased and excited because one is in a situation that they love or have been yearning for.
John's a technophile. He is like a child in a sweet shop whenever we discuss technology topics.
When she walked into this giant library for the first time, she was like a kid in a sweet shop being able to look at so many books in one place.
be so excited or nervous about that something and want to know what is going to happen next
To make someone feel more confident or happy, especially when they thought that they can't achieve what they want
Happy or cheerful; in a good mood
This expression is probably formed based on the fact that kids feel happy when they come to the sweet/candy shop.