唔該 actually has a couple of different meanings depending on the usage. In this case, it means "Excuse me" or "Pardon me" as you are trying to get that person's attention. More formally, and closer to Mandarin, you can replace 唔該 with 請問. They kind of have to same usage in this case.

The whole sentence here is "Excuse me, does your little brother like sweet dumplings?"

When you are trying to get a waiter's attention in a restaurant, you would also use 唔該. Then in this case, 唔該 also means "Excuse me" or "Pardon me."

In other cases, 唔該 can mean "Thank you" or "Please."

In other cases like,
可唔可以俾啲橙汁我呀？唔該.
It means, "Can you get me some orange juice, please?"

In the case someone hands you something (temporarily gives you that thing, like borrowing their cell phone), you would use 唔該 to say "Thank you."

However, as a receiver of a gift (permanently given to you), you would use 多謝 instead of 唔該.

Hope this is helpful. :)