She's not old enough to vote
Not old enough to drive
She will require adjusted working conditions due to her age (this is the law, she will need more frequent breaks)
She's not old enough to drink
Not old enough to sign for your shopping delivery if you buy alcohol
She's not old enough to have given birth to a child the same age as yours.
She's BARELY old enough to have sex
If she claims any form of sexual assault, you will be investigated by Social Services, because she is not legally an adult, she is legally a child.
She is very unlikely to have finished her emotional development
You will have to do a health and safety assessment because she is under 18
If you want to go out and use her for babysitting, because you are her employer you may not employ her between the hours of midnight and 4 am because she is under 18
If anything happens to the child whilst in the care of the 16 year old you will be held legally responsible, and not only will your ability to care for your child be investigated, so will your ability to care for the 16 year old.
Please, please don't employ a child.