When my dc got phones for secondary school we had a rule that we needed to know their passwords and also we could, theoretically, look at their phones at anytime. We very rarely did/do because they both seem to be doing fine but occasionally I have had a quick look to check there’s nothing inappropriate on their phones.
I’m deliberately not telling you how old dc are because I’d like to know at what point you think this switches from trying to be a cyber aware parent and trying to keep half an eye on them and when it turns to snooping and an invasion of privacy.