Using profiles will allow them to share the same account. So any items purchased can be shared by all profiles on all devices or turned off in specific profiles. The child profiles also allow for content filtering. So, yes she would keep her main profile for admin purposes and that profile can be passowrd locked. Changes can only be made from that main profile.
Profiles are account wide, so any profiles created will show up on all devices attached to the account.
You could set up separate accounts for each child, but then your daughter would have to "admin" on two devices, basically twice the work. The previous example being the solution to that (profiles shared across devices.
The only issues with profiles I have seen, or the majority of the issues anyways, are with the Google services. The profile system was developed for Nook before Google Play came along and sometimes they don't play nicely with each other. It doesn't seem to be affecting everybody though, so YMMV. The one that you should keep in mind is that Profile child filters DO NOT affect the Play Store and mom should be aware of that.
Schwa, thank you for the cogent advice. I was unaware that the Profile did not affect GoPS, but it makes sense. But the admin Profile can restrict access to that app, can it not?