I've found that even though I'm speaking the same language they know, they sometimes don't fully hear or understand, and even when they do they believe that if they change their activity slightly then the problem will be resolved without having to cease it entirely.
So before I assume ill intent, I first assess whether they understand what I'm asking. Then I ask them to evaluate if what they're doing violates my request. Usually these two things alone will resolve the situation, but in some cases I also have to make a different rule/request to get them to comply if they won't obey the simple request or rule. This might go as follows:
Me: Please stop singing, [child's name].
Child: continues singing
Me: [child's name, repeated if necessary, until I have their attention]
Child: Yes? (or looks at me)
Me: I've asked you to stop singing. Why are you still singing?
Child: I don't know. (or some variation or excuse, I didn't hear you, etc.)
Me: Well, I'd still like you to stop. Would you like me to ask you to be silent instead, or do you think you can stop singing?
Child: I will try to stop singing.
Me: Thank you.
After that, I'll remind as needed, the issue is that there's a lot going on in their head and they will absentmindedly start performing the undesired action again. At that age they aren't necessarily pushing boundaries intentionally.
Also, consider that they are bored. Engage them in discussion, sing with them, challenge them, etc. If they are living so much in their head when you are in a group setting, see if you can get them out a little and become more engaged and present. This may mean putting off the discussion about planning or finances until later and discussing something the children will find more interesting. Or relate what you're talking about to them and engage them that way.
Note that if you don't teach them to be present while they are young, they may end up being "phone zombies" as teenagers. When they aren't interested in what's going on around them, rather than trying to engage they'll simply pull back further and entertain themselves. This is learned behavior.