And how old are they?
DD is 12 and in my opinion has a lot to learn about using her phone sensibly, especially with regard to social media and commuicating with her friends that isn't to do with making plans.
I have made a point to check her phone more often during the holiday and have reminded her that dh and I do it to ensure she is protected from anything inappropriate, rather than snooping on what she is doing.
Is there much resistance from your dcs when you ask for their phone? If your dc has ever refused to hand it over, how did you resolve it?