Yes, it won’t let you book a ticket for a 5 year-old with no adults online. You’ll need to book hers via phone and have the agent link your record locator with hers (but it’ll be a separate locator since award tix can’t be on the same locator as paid ones). We used to do this all the time when we travelled with our kids when they were younger, although we do the reverse: we book our kids on the award ticket and pay cash for ours, so we can accrue the miles for the paid ticket. You might want to consider doing it that way unless there’s a reason you want to let your daughter accrue the miles instead.