No they couldn't. If we arrived at the station to pick up SD, knowing she'd text us 2 hours ago to say she was getting on the train and again after she changed trains 45 minutes earlier, and she was not there or answering her phone, we'd immediately know where she'd been in the last hour and be able to contact the last station she'd been at and get CCTV, and the last known location of her phone. There's a good chance that if she'd lost her phone, fallen suddenly ill or got lost we'd find her very quickly.
She wouldn't be tempted to go off with someone she'd just met because she'd know she was on a short timescale before it was noticed she was not where she was expected to be. If she was offered alcohol or drugs she wouldn't take them, I'd hope because she's got more sense, but if she was having a silly moment, because she would know we'd know as soon as she arrived.
There would never be a long period of time where we weren't in touch with her because by night time she is at home in her own bed.
Unless KA was texting with her child every 45 minutes, (in which case it's hardly worth letting him go and he'd be so sleep deprived as to make the whole trip torture) if he was to go missing it might be 12-24 hours before she even realised there was a problem, by which time, he could have been anywhere, and anything could have happened.
This is not anything like sending your child to stay with a relative where they get on a plane, are in sight of the air stewards/stewardesses for the whole journey and then get off to meet another responsible adult a few hours later. This is a child completely unmonitored.