We stayed at the shangri la when DS2 was 3.
If there was an activity that we thought both our children would like, eg. fish feeding, t shirt art etc. then we attended with them.
We did hire a nanny twice, you can request the same lady.
She took them to the Kids Club playground and the restaurant for lunch and another night took them to dinner and the games room/movie night.
To answer your original qu's, no one checked the ages of our children on passports. You sign your child in and out.
Your DD will be able to participate in all of the activities but will need to be accompanied by you or a nanny.
As a PP said, I wouldn't leave a young, single child there but would have felt comfortable leaving my 2 children there together if DS2 had of been old enough.