。
Please drive off as soon as your child gets out of the car.
Literal
Honorable-child [subj-が] car [obj-を] got-off [conditional-たら] immediately car [obj-を] put-out [request-ください].
おりたら (from 降りる 'to get off/out') + すぐに 'immediately' sets up the conditional temporal sequence. 車を出す 'to move the car / pull out' is the standard phrase for departing in a vehicle. お子さん uses the polite お + 子 + さん for someone else's child. Practical school drop-off instructions.