I often did this when my 3 kids were much younger by booking them on domestic flights as if they were age 15+ when I had to book a separate PNR (usually me on revenue, kids on miles). You can always add their SM number after the trip. This way I could check them in online and avoid airport hassles as we never checked bags. I never once had a problem. For international it was a pain having to scan their passport at a kiosk to get their boarding passes.