But if they're new Christmas presents, they're new and exciting and this will wear off.
Dd2 (age 10yo) got her first phone for Christmas. She hardly was parted from it for the first week, and did indeed take it to a friend's house. The first time she went out without it, she entreated me to look after it carefully. By now she isn't too bothered, and might take it with her, but would be more interested in doing other things.
Dd1(13yo) has a friend round regularly and they do use their phones as part of their games. It is a big thing in their socialising. They'll send texts across the room for private jokes, or things they've seen. But it is only part of their friendship, and while they are using their phones they're also chatting and playing. For dd1, who can struggle socially, it is a great way of interacting, if she finds something getting difficult, she can retreat into either saying what she wants by text, or play a game and have a break without making a big thing of it.
It's also a useful way for them to be able to communicate with you as parents without their friend necessarily knowing. I have a code with dd1 that she can use (and will develop one with dd2). So if her friend wants her to stay late, and she doesn't want to, she texts me a particular word before she asks-so as far as her friend knows, she's asked and I've said no.
Also code for if her and friend can't agree on something, and I can "just happen" to come up (and offer a drink or something) and smooth it over, and other things like that.
The phone has given dd1 a lot of confidence in making friends, because she feels on an even mark with them, rather than being a tag along afterthought.
From my point of view, I'd have no problem with my dc being asked not to spend time playing on the phone. Or told "you've had long enough, go and do something else". Wouldn't mind you asking them to leave it in the bag as long as they knew that if they needed to contact me then they could. Would not want you taking it off them at the door. If that was your stance I'd rather you asked them to leave it at home before they came. It feels terribly heavy handed-like confiscating it.