It almost certainly is not going to be legal for you to search your daughter’s friends’ devices, and depending on whether your daughter is emancipated, it may not be legal to search hers either. I’m assuming that you mean the device is locked and you want to force your way in to have a look. That’s not easy, especially if the device is encrypted, which is going to be the case for most iPhones and can be done with Android phones as well. If they are encrypted with a passcode, it’s going to be very difficult. Law enforcement agencies have ran into this issue. If it’s an iPhone with iOS8 that is encrypted and locked, even Apple can’t extract messages, photos, emails, etc. Security measures exists to stop just what you are trying to do.
A second issue is whether you can get someone to do it. I’m sure you could find some unscrupulous person willing to give it a try, but it might be difficult to get a legitimate company to do it. After all, they have to worry about who you are and what you are doing, and whether there will be repercussions for them. I know you said to assume it is legal, but a company will not make that assumption without a firm basis for it. I seriously doubt that most data recovery firms, which typically recover data from damaged devices, are going to take this job.
You will probably need to contact a forensic lab, and find a private one that has access to equipment like Cellebrite UFED, which is rare. You might also want to look for ISFCE certification to make sure they are reputable.
I’m not sure how much this will cost you, because an ISFCE shop will almost certainly not unlock the devices unless you can prove sole ownership. I would expect it to be pretty expensive though. I’m just guessing, but I’d bet that you are looking at $500-1000, maybe more.