When my daughter was 18, she was able to check-in at hotel. Call the hotel directly (not the reservations desk) to confirm that your kid is able to check in.
When my son had to stay in Seattle near the airport overnight after an Ultimate tournament and then fly back to WI for school, I booked the room, prepaid, then called directly after I noticed the booking policy. The person on the phone said it would probably be okay. When he checked in a month later, no go. I called personally while he stood there. No go. No refund. I panicked. To their credit, they helped us find a hotel okay for an 19 year old down the road. It was a pain and stressful for both of us. The strategies above might work but don’t risk it if you will be stressed out if it goes south. We can usually find a hotel that has a favorable policy, but that was a hard lesson.
I just booked a hotel for my wife to stay at this weekend. I put in under my name because I have the reward account. I did an online check-in with a digital key sent to her phone. I haven’t asked her, but I don’t think she probably ever even talked to the front desk. I did give them her name as also staying in the room, so if she had problems they would help her.
It was an Embassy Suites, but I think several of the other Hilton hotels are doing the digital key thing. I am staying at a Hampton in October and when I made the reservation online it said the hotel offered digital keys.