Totally agree with Oomis and others that it is fine to restrict children from cell phone, but wrong to search them. However, if they are ever seen using them, then they can be permanently confiscated. (They can sign to this at beginning of year so there is no problem of gezel.)
I don’t allow my kids to have cell phone, and won’t until they are 18 and leaving home for sem or yeshiva. Parents need to monitor their kids verbal and written communication. I am not saying to eavesdrop, just to make it possible to know who they are talking to on a regular basis. If they have a cell phone, they can be talking to who knows who about who knows what, either by phone or text all the time. Kids are not old enough to have good judgment about this, and to have that freedom.